diff --git a/Sources/ComposableArchitecture/Effect.swift b/Sources/ComposableArchitecture/Effect.swift
index 2afaf5b526f1..a7c898fe2e20 100644
--- a/Sources/ComposableArchitecture/Effect.swift
+++ b/Sources/ComposableArchitecture/Effect.swift
@@ -1,10 +1,10 @@
-import Combine
+@preconcurrency import Combine
 import Foundation
 import SwiftUI
 
-public struct Effect<Action> {
+public struct Effect<Action>: Sendable {
   @usableFromInline
-  enum Operation {
+  enum Operation: Sendable {
     case none
     case publisher(AnyPublisher<Action, Never>)
     case run(TaskPriority? = nil, @Sendable (_ send: Send<Action>) async -> Void)
diff --git a/Tests/ComposableArchitectureTests/BindingLocalTests.swift b/Tests/ComposableArchitectureTests/BindingLocalTests.swift
index 4f7dd6f673d8..0eb72a1681e4 100644
--- a/Tests/ComposableArchitectureTests/BindingLocalTests.swift
+++ b/Tests/ComposableArchitectureTests/BindingLocalTests.swift
@@ -4,6 +4,7 @@
   @testable import ComposableArchitecture
 
   final class BindingLocalTests: BaseTCATestCase {
+    @MainActor
     public func testBindingLocalIsActive() {
       XCTAssertFalse(BindingLocal.isActive)
 
diff --git a/Tests/ComposableArchitectureTests/CompatibilityTests.swift b/Tests/ComposableArchitectureTests/CompatibilityTests.swift
index 6bec1258240d..781051c5b250 100644
--- a/Tests/ComposableArchitectureTests/CompatibilityTests.swift
+++ b/Tests/ComposableArchitectureTests/CompatibilityTests.swift
@@ -6,6 +6,7 @@ final class CompatibilityTests: BaseTCATestCase {
   // Actions can be re-entrantly sent into the store if an action is sent that holds an object
   // which sends an action on deinit. In order to prevent a simultaneous access exception for this
   // case we need to use `withExtendedLifetime` on the buffered actions when clearing them out.
+  @MainActor
   func testCaseStudy_ActionReentranceFromClearedBufferCausingDeinitAction() {
     let cancelID = UUID()
 
@@ -78,6 +79,7 @@ final class CompatibilityTests: BaseTCATestCase {
   // In particular, this means that in the implementation of `Store.send` we need to flip
   // `isSending` to false _after_ the store's state mutation is made so that re-entrant actions
   // are buffered rather than immediately handled.
+  @MainActor
   func testCaseStudy_ActionReentranceFromStateObservation() {
     var cancellables: Set<AnyCancellable> = []
     defer { _ = cancellables }
diff --git a/Tests/ComposableArchitectureTests/CurrentValueRelayTests.swift b/Tests/ComposableArchitectureTests/CurrentValueRelayTests.swift
index 56c0708e653f..4bf45d8a2ea8 100644
--- a/Tests/ComposableArchitectureTests/CurrentValueRelayTests.swift
+++ b/Tests/ComposableArchitectureTests/CurrentValueRelayTests.swift
@@ -15,7 +15,7 @@
 
       await withTaskGroup(of: Void.self) { group in
         for index in 1...1_000 {
-          group.addTask {
+          group.addTask { @Sendable in
             subject.send(index)
           }
         }
diff --git a/Tests/ComposableArchitectureTests/EffectTests.swift b/Tests/ComposableArchitectureTests/EffectTests.swift
index d821a5bb352b..a277d446998c 100644
--- a/Tests/ComposableArchitectureTests/EffectTests.swift
+++ b/Tests/ComposableArchitectureTests/EffectTests.swift
@@ -47,11 +47,11 @@ final class EffectTests: BaseTCATestCase {
   }
 
   func testConcatenateOneEffect() async {
-    await withMainSerialExecutor {
+    await withMainSerialExecutor { [mainQueue] in
       let values = LockIsolated<[Int]>([])
 
       let effect = Effect<Int>.concatenate(
-        .publisher { Just(1).delay(for: 1, scheduler: self.mainQueue) }
+        .publisher { Just(1).delay(for: 1, scheduler: mainQueue) }
       )
 
       let task = Task {
@@ -62,10 +62,10 @@ final class EffectTests: BaseTCATestCase {
 
       XCTAssertEqual(values.value, [])
 
-      await self.mainQueue.advance(by: 1)
+      await mainQueue.advance(by: 1)
       XCTAssertEqual(values.value, [1])
 
-      await self.mainQueue.run()
+      await mainQueue.run()
       XCTAssertEqual(values.value, [1])
 
       await task.value
diff --git a/Tests/ComposableArchitectureTests/Reducers/BindingReducerTests.swift b/Tests/ComposableArchitectureTests/Reducers/BindingReducerTests.swift
index 77f1722074b4..0e8c63c75850 100644
--- a/Tests/ComposableArchitectureTests/Reducers/BindingReducerTests.swift
+++ b/Tests/ComposableArchitectureTests/Reducers/BindingReducerTests.swift
@@ -47,6 +47,7 @@ final class BindingTests: BaseTCATestCase {
     )
   }
 
+  @MainActor
   func testViewEquality() {
     struct Feature: Reducer {
       struct State: Equatable {
diff --git a/Tests/ComposableArchitectureTests/ScopeCacheTests.swift b/Tests/ComposableArchitectureTests/ScopeCacheTests.swift
index 4ccebb742855..e4b288a95b65 100644
--- a/Tests/ComposableArchitectureTests/ScopeCacheTests.swift
+++ b/Tests/ComposableArchitectureTests/ScopeCacheTests.swift
@@ -35,6 +35,7 @@ final class ScopeCacheTests: BaseTCATestCase {
     store.send(.child(.dismiss))
   }
 
+  @MainActor
   func testOptionalScope_CachedStore() {
     #if DEBUG
       let store = StoreOf<Feature>(initialState: Feature.State(child: Feature.State())) {
@@ -46,6 +47,7 @@ final class ScopeCacheTests: BaseTCATestCase {
     #endif
   }
 
+  @MainActor
   func testOptionalScope_StoreIfLet() {
     #if DEBUG
       let store = StoreOf<Feature>(initialState: Feature.State(child: Feature.State())) {
@@ -62,6 +64,7 @@ final class ScopeCacheTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testOptionalScope_StoreIfLet_UncachedStore() {
     let store = StoreOf<Feature>(initialState: Feature.State(child: Feature.State())) {
     }
@@ -93,6 +96,7 @@ final class ScopeCacheTests: BaseTCATestCase {
     }
   }
 
+  @MainActor
   func testIdentifiedArrayScope_CachedStore() {
     #if DEBUG
       let store = StoreOf<Feature>(initialState: Feature.State(rows: [Feature.State()])) {
@@ -108,6 +112,7 @@ final class ScopeCacheTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testIdentifiedArrayScope_UncachedStore() {
     let store = StoreOf<Feature>(initialState: Feature.State(rows: [Feature.State()])) {
       Feature()
diff --git a/Tests/ComposableArchitectureTests/SharedTests.swift b/Tests/ComposableArchitectureTests/SharedTests.swift
index 9622697ca12e..adf7ca243d39 100644
--- a/Tests/ComposableArchitectureTests/SharedTests.swift
+++ b/Tests/ComposableArchitectureTests/SharedTests.swift
@@ -96,10 +96,11 @@ final class SharedTests: XCTestCase {
     XCTAssertEqual(store.state.sharedCount, 2)
   }
 
+  @MainActor
   func testMultiSharing() async {
     @Shared(Stats()) var stats
 
-    let store = await TestStore(
+    let store = TestStore(
       initialState: SharedFeature.State(
         profile: Shared(Profile(stats: $stats)),
         sharedCount: Shared(0),
@@ -721,7 +722,7 @@ final class SharedTests: XCTestCase {
 
   func testSharedDefaults_Used() {
     let didAccess = LockIsolated(false)
-    let logDefault: () -> Bool = {
+    let logDefault: @Sendable () -> Bool = {
       didAccess.setValue(true)
       return true
     }
@@ -732,7 +733,7 @@ final class SharedTests: XCTestCase {
 
   func testSharedDefaults_Unused() {
     let didAccess = LockIsolated(false)
-    let logDefault: () -> Bool = {
+    let logDefault: @Sendable () -> Bool = {
       didAccess.setValue(true)
       return true
     }
@@ -761,7 +762,7 @@ final class SharedTests: XCTestCase {
   func testSharedOverrideDefault() {
     let accessedActive1 = LockIsolated(false)
     let accessedDefault = LockIsolated(false)
-    let logDefault: () -> Bool = {
+    let logDefault: @Sendable () -> Bool = {
       accessedDefault.setValue(true)
       return true
     }
@@ -796,7 +797,7 @@ final class SharedTests: XCTestCase {
   func testSharedReaderOverrideDefault() {
     let accessedActive1 = LockIsolated(false)
     let accessedDefault = LockIsolated(false)
-    let logDefault: () -> Bool = {
+    let logDefault: @Sendable () -> Bool = {
       accessedDefault.setValue(true)
       return true
     }
@@ -1141,7 +1142,7 @@ private struct SimpleFeature {
 }
 
 @Perceptible
-class SharedObject {
+class SharedObject: @unchecked Sendable {
   var count = 0
 }
 
@@ -1244,7 +1245,7 @@ extension PersistenceReaderKey where Self == PersistenceKeyDefault<AppStorageKey
     PersistenceKeyDefault(.appStorage("isOn"), false)
   }
 
-  static func isActive(default keyDefault: @escaping () -> Bool) -> Self {
+  static func isActive(default keyDefault: @escaping @Sendable () -> Bool) -> Self {
     PersistenceKeyDefault(.appStorage("isActive"), keyDefault())
   }
 }
diff --git a/Tests/ComposableArchitectureTests/StoreFilterTests.swift b/Tests/ComposableArchitectureTests/StoreFilterTests.swift
index 66c5a271061e..5781116a6cc3 100644
--- a/Tests/ComposableArchitectureTests/StoreFilterTests.swift
+++ b/Tests/ComposableArchitectureTests/StoreFilterTests.swift
@@ -3,6 +3,7 @@ import Combine
 import XCTest
 
 final class StoreInvalidationTests: BaseTCATestCase {
+  @MainActor
   func testInvalidation() {
     var cancellables: Set<AnyCancellable> = []
 
diff --git a/Tests/ComposableArchitectureTests/StoreLifetimeTests.swift b/Tests/ComposableArchitectureTests/StoreLifetimeTests.swift
index 79a6add3c1b9..065f94a74dcb 100644
--- a/Tests/ComposableArchitectureTests/StoreLifetimeTests.swift
+++ b/Tests/ComposableArchitectureTests/StoreLifetimeTests.swift
@@ -4,6 +4,7 @@ import XCTest
 
 final class StoreLifetimeTests: BaseTCATestCase {
   @available(*, deprecated)
+  @MainActor
   func testStoreCaching() {
     let grandparentStore = Store(initialState: Grandparent.State()) {
       Grandparent()
@@ -21,6 +22,7 @@ final class StoreLifetimeTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testStoreInvalidation() {
     let grandparentStore = Store(initialState: Grandparent.State()) {
       Grandparent()
@@ -48,6 +50,7 @@ final class StoreLifetimeTests: BaseTCATestCase {
   }
 
   #if DEBUG
+    @MainActor
     func testStoreDeinit() {
       Logger.shared.isEnabled = true
       do {
diff --git a/Tests/ComposableArchitectureTests/StoreTests.swift b/Tests/ComposableArchitectureTests/StoreTests.swift
index 2dd3f0182031..50037cd87877 100644
--- a/Tests/ComposableArchitectureTests/StoreTests.swift
+++ b/Tests/ComposableArchitectureTests/StoreTests.swift
@@ -1,4 +1,4 @@
-import Combine
+@preconcurrency import Combine
 @_spi(Internals) import ComposableArchitecture
 import XCTest
 
@@ -47,6 +47,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testScopedStoreReceivesUpdatesFromParent() {
     let counterReducer = Reduce<Int, Void>({ state, _ in
       state += 1
@@ -71,6 +72,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testParentStoreReceivesUpdatesFromChild() {
     let counterReducer = Reduce<Int, Void>({ state, _ in
       state += 1
@@ -95,6 +97,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testScopeCallCount_OneLevel_NoSubscription() {
     var numCalls1 = 0
     let store = Store<Int, Void>(initialState: 0) {}
@@ -112,6 +115,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testScopeCallCount_OneLevel_Subscribing() {
     var numCalls1 = 0
     let store = Store<Int, Void>(initialState: 0) {}
@@ -130,6 +134,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testScopeCallCount_TwoLevels_Subscribing() {
     var numCalls1 = 0
     var numCalls2 = 0
@@ -158,6 +163,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testScopeCallCount_ThreeLevels_ViewStoreSubscribing() {
     var numCalls1 = 0
     var numCalls2 = 0
@@ -280,6 +286,7 @@ final class StoreTests: BaseTCATestCase {
     XCTAssertEqual(values, [1, 2, 3, 4])
   }
 
+  @MainActor
   func testLotsOfSynchronousActions() {
     enum Action { case incr, noop }
     let reducer = Reduce<Int, Action>({ state, action in
@@ -350,6 +357,7 @@ final class StoreTests: BaseTCATestCase {
     XCTAssertEqual(outputs, [nil, 1, nil, 1, nil, 1, nil])
   }
 
+  @MainActor
   func testIfLetTwo() {
     let parentStore = Store(initialState: 0) {
       Reduce<Int?, Bool> { state, action in
@@ -382,6 +390,7 @@ final class StoreTests: BaseTCATestCase {
       .store(in: &self.cancellables)
   }
 
+  @MainActor
   func testActionQueuing() async {
     let subject = PassthroughSubject<Void, Never>()
 
@@ -391,7 +400,7 @@ final class StoreTests: BaseTCATestCase {
       case doIncrement
     }
 
-    let store = await TestStore(initialState: 0) {
+    let store = TestStore(initialState: 0) {
       Reduce<Int, Action> { state, action in
         switch action {
         case .incrementTapped:
@@ -420,6 +429,7 @@ final class StoreTests: BaseTCATestCase {
     subject.send(completion: .finished)
   }
 
+  @MainActor
   func testCoalesceSynchronousActions() {
     let store = Store(initialState: 0) {
       Reduce<Int, Int> { state, action in
@@ -451,6 +461,7 @@ final class StoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testBufferedActionProcessing() {
     struct ChildState: Equatable {
       var count: Int?
diff --git a/Tests/ComposableArchitectureTests/TestStoreTests.swift b/Tests/ComposableArchitectureTests/TestStoreTests.swift
index 78cc900fb268..d919a1350028 100644
--- a/Tests/ComposableArchitectureTests/TestStoreTests.swift
+++ b/Tests/ComposableArchitectureTests/TestStoreTests.swift
@@ -1,4 +1,4 @@
-import Combine
+@preconcurrency import Combine
 import ComposableArchitecture
 import XCTest
 
@@ -396,6 +396,7 @@ final class TestStoreTests: BaseTCATestCase {
     }
   }
 
+  @MainActor
   func testPrepareDependenciesCalledOnce() {
     var count = 0
     let store = TestStore(initialState: 0) {
@@ -469,6 +470,7 @@ final class TestStoreTests: BaseTCATestCase {
     }
   }
 
+  @MainActor
   func testSubscribeReceiveCombineScheduler() async {
     let subject = PassthroughSubject<Void, Never>()
     let scheduler = DispatchQueue.test
@@ -482,7 +484,7 @@ final class TestStoreTests: BaseTCATestCase {
       case start
     }
 
-    let store = await TestStore(initialState: State()) {
+    let store = TestStore(initialState: State()) {
       Reduce<State, Action> { state, action in
         switch action {
         case .start:
@@ -659,7 +661,7 @@ final class TestStoreTests: BaseTCATestCase {
 }
 
 private struct Client: DependencyKey {
-  var fetch: () -> Int
+  var fetch: @Sendable () -> Int
   static let liveValue = Client(fetch: { 42 })
 }
 extension DependencyValues {
diff --git a/Tests/ComposableArchitectureTests/ThrottleTests.swift b/Tests/ComposableArchitectureTests/ThrottleTests.swift
index 6415ebe60f75..519a9115c751 100644
--- a/Tests/ComposableArchitectureTests/ThrottleTests.swift
+++ b/Tests/ComposableArchitectureTests/ThrottleTests.swift
@@ -1,6 +1,6 @@
 import Combine
 import ComposableArchitecture
-import XCTest
+@preconcurrency import XCTest
 
 final class EffectThrottleTests: BaseTCATestCase {
   let mainQueue = DispatchQueue.test
@@ -8,7 +8,7 @@ final class EffectThrottleTests: BaseTCATestCase {
   func testThrottleLatest_Publisher() async {
     let store = await TestStore(initialState: ThrottleFeature.State()) {
       ThrottleFeature(id: #function, latest: true)
-    } withDependencies: {
+    } withDependencies: { [mainQueue] in
       $0.mainQueue = mainQueue.eraseToAnyScheduler()
     }
 
@@ -51,7 +51,7 @@ final class EffectThrottleTests: BaseTCATestCase {
   func testThrottleLatest_Async() async {
     let store = await TestStore(initialState: ThrottleFeature.State()) {
       ThrottleFeature(id: #function, latest: true)
-    } withDependencies: {
+    } withDependencies: { [mainQueue] in
       $0.mainQueue = mainQueue.eraseToAnyScheduler()
     }
 
@@ -94,7 +94,7 @@ final class EffectThrottleTests: BaseTCATestCase {
   func testThrottleFirst_Publisher() async {
     let store = await TestStore(initialState: ThrottleFeature.State()) {
       ThrottleFeature(id: #function, latest: false)
-    } withDependencies: {
+    } withDependencies: { [mainQueue] in
       $0.mainQueue = mainQueue.eraseToAnyScheduler()
     }
 
@@ -137,7 +137,7 @@ final class EffectThrottleTests: BaseTCATestCase {
   func testThrottleAfterInterval_Publisher() async {
     let store = await TestStore(initialState: ThrottleFeature.State()) {
       ThrottleFeature(id: #function, latest: true)
-    } withDependencies: {
+    } withDependencies: { [mainQueue] in
       $0.mainQueue = mainQueue.eraseToAnyScheduler()
     }
 
@@ -158,7 +158,7 @@ final class EffectThrottleTests: BaseTCATestCase {
   func testThrottleEmitsFirstValueOnce_Publisher() async {
     let store = await TestStore(initialState: ThrottleFeature.State()) {
       ThrottleFeature(id: #function, latest: true)
-    } withDependencies: {
+    } withDependencies: { [mainQueue] in
       $0.mainQueue = mainQueue.eraseToAnyScheduler()
     }
 
diff --git a/Tests/ComposableArchitectureTests/ViewStoreTests.swift b/Tests/ComposableArchitectureTests/ViewStoreTests.swift
index 1d9956d999db..f699997f69bb 100644
--- a/Tests/ComposableArchitectureTests/ViewStoreTests.swift
+++ b/Tests/ComposableArchitectureTests/ViewStoreTests.swift
@@ -1,4 +1,4 @@
-import Combine
+@preconcurrency import Combine
 import ComposableArchitecture
 import XCTest
 
@@ -7,10 +7,11 @@ final class ViewStoreTests: BaseTCATestCase {
 
   override func setUpWithError() throws {
     try super.setUpWithError()
-    equalityChecks = 0
-    subEqualityChecks = 0
+    equalityChecks.setValue(0)
+    subEqualityChecks.setValue(0)
   }
 
+  @MainActor
   func testPublisherFirehose() {
     let store = Store<Int, Void>(initialState: 0) {}
     let viewStore = ViewStore(store, observe: { $0 })
@@ -30,6 +31,7 @@ final class ViewStoreTests: BaseTCATestCase {
   }
 
   @available(*, deprecated)
+  @MainActor
   func testEqualityChecks() {
     let store = Store<State, Void>(initialState: State()) {}
 
@@ -52,22 +54,23 @@ final class ViewStoreTests: BaseTCATestCase {
     viewStore3.publisher.substate.sink { _ in }.store(in: &self.cancellables)
     viewStore4.publisher.substate.sink { _ in }.store(in: &self.cancellables)
 
-    XCTAssertEqual(0, equalityChecks)
-    XCTAssertEqual(0, subEqualityChecks)
+    XCTAssertEqual(0, equalityChecks.value)
+    XCTAssertEqual(0, subEqualityChecks.value)
     viewStore4.send(())
-    XCTAssertEqual(4, equalityChecks)
-    XCTAssertEqual(4, subEqualityChecks)
+    XCTAssertEqual(4, equalityChecks.value)
+    XCTAssertEqual(4, subEqualityChecks.value)
     viewStore4.send(())
-    XCTAssertEqual(8, equalityChecks)
-    XCTAssertEqual(8, subEqualityChecks)
+    XCTAssertEqual(8, equalityChecks.value)
+    XCTAssertEqual(8, subEqualityChecks.value)
     viewStore4.send(())
-    XCTAssertEqual(12, equalityChecks)
-    XCTAssertEqual(12, subEqualityChecks)
+    XCTAssertEqual(12, equalityChecks.value)
+    XCTAssertEqual(12, subEqualityChecks.value)
     viewStore4.send(())
-    XCTAssertEqual(16, equalityChecks)
-    XCTAssertEqual(16, subEqualityChecks)
+    XCTAssertEqual(16, equalityChecks.value)
+    XCTAssertEqual(16, subEqualityChecks.value)
   }
 
+  @MainActor
   func testAccessViewStoreStateInPublisherSink() {
     let reducer = Reduce<Int, Void> { count, _ in
       count += 1
@@ -90,6 +93,7 @@ final class ViewStoreTests: BaseTCATestCase {
     XCTAssertEqual([0, 1, 2, 3], results)
   }
 
+  @MainActor
   func testWillSet() {
     let reducer = Reduce<Int, Void> { count, _ in
       count += 1
@@ -114,6 +118,7 @@ final class ViewStoreTests: BaseTCATestCase {
     XCTAssertEqual([0, 1, 2], results)
   }
 
+  @MainActor
   func testPublisherOwnsViewStore() {
     let reducer = Reduce<Int, Void> { count, _ in
       count += 1
@@ -131,6 +136,7 @@ final class ViewStoreTests: BaseTCATestCase {
     XCTAssertEqual(results, [0, 1])
   }
 
+  @MainActor
   func testStorePublisherSubscriptionOrder() {
     let store = Store<Int, Void>(initialState: 0) {
       Reduce { state, _ in
@@ -299,7 +305,7 @@ private struct State: Equatable {
   var substate = Substate()
 
   static func == (lhs: Self, rhs: Self) -> Bool {
-    equalityChecks += 1
+    equalityChecks.withValue { $0 += 1 }
     return lhs.substate == rhs.substate
   }
 }
@@ -308,10 +314,10 @@ private struct Substate: Equatable {
   var name = "Blob"
 
   static func == (lhs: Self, rhs: Self) -> Bool {
-    subEqualityChecks += 1
+    subEqualityChecks.withValue { $0 += 1 }
     return lhs.name == rhs.name
   }
 }
 
-private var equalityChecks = 0
-private var subEqualityChecks = 0
+private let equalityChecks = LockIsolated(0)
+private let subEqualityChecks = LockIsolated(0)