diff --git a/CLI/Sources/KnitCodeGen/UnitTestSourceFile.swift b/CLI/Sources/KnitCodeGen/UnitTestSourceFile.swift
index 3fefd63..be365d2 100644
--- a/CLI/Sources/KnitCodeGen/UnitTestSourceFile.swift
+++ b/CLI/Sources/KnitCodeGen/UnitTestSourceFile.swift
@@ -138,6 +138,7 @@ public enum UnitTestSourceFile {
 
     private static func makeCollectionAssert() throws -> FunctionDeclSyntax {
         let string: SyntaxNodeString = #"""
+        @MainActor
         func assertCollectionResolves<T>(
             _ type: T.Type,
             count expectedCount: Int,
diff --git a/CLI/Tests/KnitCodeGenTests/ConfigurationSetTests.swift b/CLI/Tests/KnitCodeGenTests/ConfigurationSetTests.swift
index 7ce132c..6102ed8 100644
--- a/CLI/Tests/KnitCodeGenTests/ConfigurationSetTests.swift
+++ b/CLI/Tests/KnitCodeGenTests/ConfigurationSetTests.swift
@@ -140,6 +140,7 @@ final class ConfigurationSetTests: XCTestCase {
                         line: line
                     )
                 }
+                @MainActor
                 func assertCollectionResolves<T>(
                     _ type: T.Type,
                     count expectedCount: Int,
diff --git a/Example/KnitExample/KnitExampleAssembly.swift b/Example/KnitExample/KnitExampleAssembly.swift
index 3d1206a..a9debe6 100644
--- a/Example/KnitExample/KnitExampleAssembly.swift
+++ b/Example/KnitExample/KnitExampleAssembly.swift
@@ -36,8 +36,22 @@ final class KnitExampleAssembly: ModuleAssembly {
             ClosureService(closure: arg1)
         }
 
+        container.register(
+            MainActorService.self,
+            mainActorFactory: { _ in
+                MainActorService()
+            }
+        )
+
         container.autoregisterIntoCollection(ExampleService.self, initializer: ExampleService.init)
 
+        container.registerIntoCollection(
+            MainActorService.self,
+            factory: { _ in
+                MainActorService()
+            }
+        )
+
         #if DEBUG
         container.autoregister(DebugService.self, initializer: DebugService.init)
         #endif
@@ -81,3 +95,10 @@ final class ClosureService {
 }
 
 struct DebugService { }
+
+@MainActor
+final class MainActorService {
+    init() { }
+
+    var title: String { "Example String" }
+}
diff --git a/Sources/Knit/ServiceCollection/Container+ServiceCollection.erb b/Sources/Knit/ServiceCollection/Container+ServiceCollection.erb
index 87c34c5..70330e9 100644
--- a/Sources/Knit/ServiceCollection/Container+ServiceCollection.erb
+++ b/Sources/Knit/ServiceCollection/Container+ServiceCollection.erb
@@ -33,9 +33,17 @@ extension Container {
     @discardableResult
     public func registerIntoCollection<Service>(
         _ service: Service.Type,
-        factory: @escaping (Resolver) -> Service
+        factory: @escaping @MainActor (Resolver) -> Service
     ) -> ServiceEntry<Service> {
-        self.register(service, name: makeUniqueCollectionRegistrationName(), factory: factory)
+        self.register(
+            service,
+            name: makeUniqueCollectionRegistrationName(),
+            factory: { resolver in
+                MainActor.assumeIsolated {
+                    return factory(resolver)
+                }
+            }
+        )
     }
 
     /// Registers a service factory into a collection.
diff --git a/Sources/Knit/ServiceCollection/Container+ServiceCollection.swift b/Sources/Knit/ServiceCollection/Container+ServiceCollection.swift
index a8f78e6..29bb905 100644
--- a/Sources/Knit/ServiceCollection/Container+ServiceCollection.swift
+++ b/Sources/Knit/ServiceCollection/Container+ServiceCollection.swift
@@ -32,9 +32,17 @@ extension Container {
     @discardableResult
     public func registerIntoCollection<Service>(
         _ service: Service.Type,
-        factory: @escaping (Resolver) -> Service
+        factory: @escaping @MainActor (Resolver) -> Service
     ) -> ServiceEntry<Service> {
-        self.register(service, name: makeUniqueCollectionRegistrationName(), factory: factory)
+        self.register(
+            service,
+            name: makeUniqueCollectionRegistrationName(),
+            factory: { resolver in
+                MainActor.assumeIsolated {
+                    return factory(resolver)
+                }
+            }
+        )
     }
 
     /// Registers a service factory into a collection.
diff --git a/Sources/Knit/ServiceCollection/Resolver+ServiceCollection.swift b/Sources/Knit/ServiceCollection/Resolver+ServiceCollection.swift
index 628b24d..5e56fe4 100644
--- a/Sources/Knit/ServiceCollection/Resolver+ServiceCollection.swift
+++ b/Sources/Knit/ServiceCollection/Resolver+ServiceCollection.swift
@@ -21,6 +21,7 @@ extension Resolver {
     /// - Parameter serviceType: The service types to resolve.
     /// - Returns: A ``ServiceCollection`` containing all registered services,
     ///            or an empty collection if no services were registered.
+    @MainActor
     public func resolveCollection<Service>(_ serviceType: Service.Type) -> ServiceCollection<Service> {
         resolve(ServiceCollection<Service>.self) ?? .init(parent: nil, entries: [])
     }
diff --git a/Tests/KnitTests/ServiceCollectorTests.swift b/Tests/KnitTests/ServiceCollectorTests.swift
index d24380b..900bd3e 100644
--- a/Tests/KnitTests/ServiceCollectorTests.swift
+++ b/Tests/KnitTests/ServiceCollectorTests.swift
@@ -53,6 +53,7 @@ final class ServiceCollectorTests: XCTestCase {
 
     // MARK: - Tests - registerIntoCollection
 
+    @MainActor
     func test_registerIntoCollection() {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -78,6 +79,7 @@ final class ServiceCollectorTests: XCTestCase {
         )
     }
 
+    @MainActor
     func test_registerIntoCollection_emptyWithBehavior() {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -86,6 +88,7 @@ final class ServiceCollectorTests: XCTestCase {
         XCTAssertEqual(collection.entries.count, 0)
     }
 
+    @MainActor
     func test_registerIntoCollection_emptyWithoutBehavior() {
         let container = Container()
 
@@ -95,6 +98,7 @@ final class ServiceCollectorTests: XCTestCase {
 
     /// ``ServiceCollector`` shouldn't preclude users from registering their own separate ``Array<Service>``.
     /// A conflict here would be confusing and surprising to the user.
+    @MainActor
     func test_registerIntoCollection_doesntConflictWithArray() throws {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -116,6 +120,7 @@ final class ServiceCollectorTests: XCTestCase {
         XCTAssert(array.first is ServiceB)
     }
 
+    @MainActor
     func test_registerIntoCollection_doesntImplicitlyAggregateInstances() throws {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -137,6 +142,7 @@ final class ServiceCollectorTests: XCTestCase {
         XCTAssert(container.resolve(ServiceProtocol.self) is ServiceB)
     }
 
+    @MainActor
     func test_registerIntoCollection_allowsDuplicates() {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -156,6 +162,7 @@ final class ServiceCollectorTests: XCTestCase {
 
     // MARK: - Tests - autoregisterIntoCollection
 
+    @MainActor
     func test_autoregisterIntoCollection() {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -172,6 +179,7 @@ final class ServiceCollectorTests: XCTestCase {
     }
 
     // High-arity overloads are generated by a script. Ensure they work as expected.
+    @MainActor
     func test_autoregisterIntoCollection_highArityOverloads() {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -195,6 +203,7 @@ final class ServiceCollectorTests: XCTestCase {
 
     // MARK: - Tests - Object Scopes
 
+    @MainActor
     func test_registerIntoCollection_supportsTransientScopedObjects() throws {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -214,6 +223,7 @@ final class ServiceCollectorTests: XCTestCase {
         XCTAssert(instance1 !== instance2)
     }
 
+    @MainActor
     func test_registerIntoCollection_supportsContainerScopedObjects() throws {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -233,6 +243,7 @@ final class ServiceCollectorTests: XCTestCase {
         XCTAssert(instance1 === instance2)
     }
 
+    @MainActor
     func test_registerIntoCollection_supportsWeakScopedObjects() throws {
         let container = Container()
         container.addBehavior(ServiceCollector())
@@ -264,6 +275,7 @@ final class ServiceCollectorTests: XCTestCase {
         XCTAssertEqual(factoryCallCount, 2)
     }
 
+    @MainActor
     func test_parentChildContainersWithAssemblers() {
         let parent = ModuleAssembler([AssemblyA()])
         let child = ModuleAssembler(parent: parent, [AssemblyB()])
@@ -288,6 +300,7 @@ final class ServiceCollectorTests: XCTestCase {
         
     }
 
+    @MainActor
     func test_childWithEmptyParent() {
         let parent = ModuleAssembler([AssemblyC()])
         let child = ModuleAssembler(parent: parent, [AssemblyB()])
@@ -303,7 +316,8 @@ final class ServiceCollectorTests: XCTestCase {
             1
         )
     }
-    
+
+    @MainActor
     func test_emptyChildWithParent() {
         let parent = ModuleAssembler([AssemblyB()])
         let child = ModuleAssembler(parent: parent, [AssemblyC()])
@@ -321,6 +335,7 @@ final class ServiceCollectorTests: XCTestCase {
         )
     }
 
+    @MainActor
     func test_grandparentRelationship() {
         let grandParent = ModuleAssembler([AssemblyA()])
         let parent = ModuleAssembler(parent: grandParent, [AssemblyC()])