diff --git a/Tests/NIOAsyncRuntimeTests/MultiThreadedEventLoopGroupTests.swift b/Tests/NIOAsyncRuntimeTests/MultiThreadedEventLoopGroupTests.swift
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+++ b/Tests/NIOAsyncRuntimeTests/MultiThreadedEventLoopGroupTests.swift
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+//===----------------------------------------------------------------------===//
+//
+// Copyright (c) 2025 PassiveLogic, Inc.
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import NIOConcurrencyHelpers
+import Testing
+
+@testable import NIOAsyncRuntime
+@testable import NIOCore
+
+// NOTE: These tests are copied and adapted from NIOPosixTests.EventLoopTest
+// They have been modified to use async running, among other things.
+
+@Suite("MultiThreadedEventLoopGroupTests", .serialized, .timeLimit(.minutes(1)))
+final class MultiThreadedEventLoopGroupTests {
+  @Test
+  func testLotsOfMixedImmediateAndScheduledTasks() async throws {
+    let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+    defer {
+      #expect(throws: Never.self) {
+        try group.syncShutdownGracefully()
+      }
+    }
+
+    let eventLoop = group.next()
+    struct Counter: Sendable {
+      private var _submitCount = NIOLockedValueBox(0)
+      var submitCount: Int {
+        get { self._submitCount.withLockedValue { $0 } }
+        nonmutating set { self._submitCount.withLockedValue { $0 = newValue } }
+      }
+      private var _scheduleCount = NIOLockedValueBox(0)
+      var scheduleCount: Int {
+        get { self._scheduleCount.withLockedValue { $0 } }
+        nonmutating set { self._scheduleCount.withLockedValue { $0 = newValue } }
+      }
+    }
+
+    let achieved = Counter()
+    var immediateTasks = [EventLoopFuture<Void>]()
+    var scheduledTasks = [Scheduled<Void>]()
+    for _ in (0..<100_000) {
+      if Bool.random() {
+        let task = eventLoop.submit {
+          achieved.submitCount += 1
+        }
+        immediateTasks.append(task)
+      }
+      if Bool.random() {
+        let task = eventLoop.scheduleTask(in: .microseconds(10)) {
+          achieved.scheduleCount += 1
+        }
+        scheduledTasks.append(task)
+      }
+    }
+
+    let submitCount = try await EventLoopFuture.whenAllSucceed(immediateTasks, on: eventLoop).map({
+      _ in
+      achieved.submitCount
+    }).get()
+    #expect(submitCount == achieved.submitCount)
+
+    let scheduleCount = try await EventLoopFuture.whenAllSucceed(
+      scheduledTasks.map { $0.futureResult },
+      on: eventLoop
+    )
+    .map({ _ in
+      achieved.scheduleCount
+    }).get()
+    #expect(scheduleCount == scheduledTasks.count)
+  }
+
+  @Test
+  func testLotsOfMixedImmediateAndScheduledTasksFromEventLoop() async throws {
+    let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+    defer {
+      #expect(throws: Never.self) {
+        try group.syncShutdownGracefully()
+      }
+    }
+
+    let eventLoop = group.next()
+    struct Counter: Sendable {
+      private var _submitCount = NIOLockedValueBox(0)
+      var submitCount: Int {
+        get { self._submitCount.withLockedValue { $0 } }
+        nonmutating set { self._submitCount.withLockedValue { $0 = newValue } }
+      }
+      private var _scheduleCount = NIOLockedValueBox(0)
+      var scheduleCount: Int {
+        get { self._scheduleCount.withLockedValue { $0 } }
+        nonmutating set { self._scheduleCount.withLockedValue { $0 = newValue } }
+      }
+    }
+
+    let achieved = Counter()
+    let (immediateTasks, scheduledTasks) = try await eventLoop.submit {
+      var immediateTasks = [EventLoopFuture<Void>]()
+      var scheduledTasks = [Scheduled<Void>]()
+      for _ in (0..<100_000) {
+        if Bool.random() {
+          let task = eventLoop.submit {
+            achieved.submitCount += 1
+          }
+          immediateTasks.append(task)
+        }
+        if Bool.random() {
+          let task = eventLoop.scheduleTask(in: .microseconds(10)) {
+            achieved.scheduleCount += 1
+          }
+          scheduledTasks.append(task)
+        }
+      }
+      return (immediateTasks, scheduledTasks)
+    }.get()
+
+    let submitCount = try await EventLoopFuture.whenAllSucceed(immediateTasks, on: eventLoop)
+      .map({ _ in
+        achieved.submitCount
+      }).get()
+    #expect(submitCount == achieved.submitCount)
+
+    let scheduleCount = try await EventLoopFuture.whenAllSucceed(
+      scheduledTasks.map { $0.futureResult },
+      on: eventLoop
+    )
+    .map({ _ in
+      achieved.scheduleCount
+    }).get()
+    #expect(scheduleCount == scheduledTasks.count)
+  }
+
+  @Test
+  func testImmediateTasksDontGetStuck() async throws {
+    let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+    defer {
+      #expect(throws: Never.self) {
+        try group.syncShutdownGracefully()
+      }
+    }
+
+    let eventLoop = group.next()
+    let testEventLoop = MultiThreadedEventLoopGroup.singleton.any()
+
+    let longWait = TimeAmount.seconds(60)
+    let failDeadline = NIODeadline.now() + longWait
+    let (immediateTasks, scheduledTask) = try await eventLoop.submit {
+      // Submit over the 4096 immediate tasks, and some scheduled tasks
+      // with expiry deadline in (nearish) future.
+      // We want to make sure immediate tasks, even those that don't fit
+      // in the first batch, don't get stuck waiting for scheduled task
+      // expiry
+      let immediateTasks = (0..<5000).map { _ in
+        eventLoop.submit {}.hop(to: testEventLoop)
+      }
+      let scheduledTask = eventLoop.scheduleTask(in: longWait) {
+      }
+
+      return (immediateTasks, scheduledTask)
+    }.get()
+
+    // The immediate tasks should all succeed ~immediately.
+    // We're testing for a case where the EventLoop gets confused
+    // into waiting for the scheduled task expiry to complete
+    // some immediate tasks.
+    _ = try await EventLoopFuture.whenAllSucceed(immediateTasks, on: testEventLoop).get()
+    #expect(.now() < failDeadline)
+
+    scheduledTask.cancel()
+  }
+
+  @Test
+  func testInEventLoopABAProblem() async throws {
+    // Older SwiftNIO versions had a bug here, they held onto `pthread_t`s for ever (which is illegal) and then
+    // used `pthread_equal(pthread_self(), myPthread)`. `pthread_equal` just compares the pointer values which
+    // means there's an ABA problem here. This test checks that we don't suffer from that issue now.
+    let allELs: NIOLockedValueBox<[any EventLoop]> = NIOLockedValueBox([])
+
+    for _ in 0..<100 {
+      let group = MultiThreadedEventLoopGroup(numberOfThreads: 4)
+      defer {
+        #expect(throws: Never.self) {
+          try group.syncShutdownGracefully()
+        }
+      }
+      for loop in group.makeIterator() {
+        try! await loop.submit {
+          allELs.withLockedValue { allELs in
+            #expect(loop.inEventLoop)
+            for otherEL in allELs {
+              #expect(
+                !otherEL.inEventLoop,
+                "should only be in \(loop) but turns out also in \(otherEL)"
+              )
+            }
+            allELs.append(loop)
+          }
+        }.get()
+      }
+    }
+  }
+}