diff --git a/pitest/src/main/java/org/pitest/mutationtest/execute/MutationTestWorker.java b/pitest/src/main/java/org/pitest/mutationtest/execute/MutationTestWorker.java
index 1f257f624..0cb9480fe 100644
--- a/pitest/src/main/java/org/pitest/mutationtest/execute/MutationTestWorker.java
+++ b/pitest/src/main/java/org/pitest/mutationtest/execute/MutationTestWorker.java
@@ -38,6 +38,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.ConcurrentLinkedDeque;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import java.util.stream.Collectors;
@@ -168,11 +169,11 @@ private static Container createNewContainer() {
     return new UnContainer() {
       @Override
       public List<TestResult> execute(final TestUnit group) {
-        final List<TestResult> results = new ArrayList<>();
+        final Collection<TestResult> results = new ConcurrentLinkedDeque<>();
         final ExitingResultCollector rc = new ExitingResultCollector(
             new ConcreteResultCollector(results));
         group.execute(rc);
-        return results;
+        return new ArrayList<>(results);
       }
     };
   }
diff --git a/pitest/src/main/java/org/pitest/testapi/ResultCollector.java b/pitest/src/main/java/org/pitest/testapi/ResultCollector.java
index 8d7806395..b23739701 100644
--- a/pitest/src/main/java/org/pitest/testapi/ResultCollector.java
+++ b/pitest/src/main/java/org/pitest/testapi/ResultCollector.java
@@ -14,6 +14,11 @@
  */
 package org.pitest.testapi;
 
+/**
+ * All implementations of this interface should be thread-safe, as the methods can be
+ * called from various threads and also from parallel running tests, depending on the used
+ * test engine.
+ */
 public interface ResultCollector {
 
   void notifyEnd(Description description, Throwable t);
diff --git a/pitest/src/main/java/org/pitest/testapi/TestUnitExecutionListener.java b/pitest/src/main/java/org/pitest/testapi/TestUnitExecutionListener.java
index e807d71db..3529329f4 100644
--- a/pitest/src/main/java/org/pitest/testapi/TestUnitExecutionListener.java
+++ b/pitest/src/main/java/org/pitest/testapi/TestUnitExecutionListener.java
@@ -8,6 +8,10 @@
  * <p>At least one of the {@code executionStarted} methods needs to be overwritten
  * by subclasses as the default implementations call each other for backwards compatibility
  * and thus would cause a stack overflow error if called.
+ *
+ * <p>All implementations of this interface should be thread-safe, as the methods can be
+ * called from various threads and also from parallel running tests, depending on the used
+ * test engine.
  */
 public interface TestUnitExecutionListener {
     default void executionStarted(Description description) {
diff --git a/pitest/src/main/java/org/pitest/testapi/execute/ExitingResultCollector.java b/pitest/src/main/java/org/pitest/testapi/execute/ExitingResultCollector.java
index 7dac22150..686a8096a 100644
--- a/pitest/src/main/java/org/pitest/testapi/execute/ExitingResultCollector.java
+++ b/pitest/src/main/java/org/pitest/testapi/execute/ExitingResultCollector.java
@@ -17,10 +17,12 @@
 import org.pitest.testapi.Description;
 import org.pitest.testapi.ResultCollector;
 
+import java.util.concurrent.atomic.AtomicBoolean;
+
 public class ExitingResultCollector implements ResultCollector {
 
   private final ResultCollector child;
-  private boolean               hadFailure = false;
+  private final AtomicBoolean hadFailure = new AtomicBoolean(false);
 
   public ExitingResultCollector(final ResultCollector child) {
     this.child = child;
@@ -38,14 +40,14 @@ public void notifyStart(final Description description) {
 
   @Override
   public boolean shouldExit() {
-    return this.hadFailure;
+    return this.hadFailure.get();
   }
 
   @Override
   public void notifyEnd(final Description description, final Throwable t) {
     this.child.notifyEnd(description, t);
     if (t != null) {
-      this.hadFailure = true;
+      this.hadFailure.set(true);
     }
 
   }
@@ -55,4 +57,4 @@ public void notifyEnd(final Description description) {
     this.child.notifyEnd(description);
   }
 
-}
\ No newline at end of file
+}
diff --git a/pitest/src/main/java/org/pitest/testapi/execute/containers/ConcreteResultCollector.java b/pitest/src/main/java/org/pitest/testapi/execute/containers/ConcreteResultCollector.java
index e45f979c6..7a007cfca 100644
--- a/pitest/src/main/java/org/pitest/testapi/execute/containers/ConcreteResultCollector.java
+++ b/pitest/src/main/java/org/pitest/testapi/execute/containers/ConcreteResultCollector.java
@@ -25,6 +25,11 @@ public final class ConcreteResultCollector implements ResultCollector {
 
   private final Collection<TestResult> feedback;
 
+  /**
+   * @param feedback the collection where to collect test results; this collection should be thread-safe
+   *                 as it can be called from various threads and also from parallel running tests,
+   *                 depending on the used test engine.
+   */
   public ConcreteResultCollector(final Collection<TestResult> feedback) {
     this.feedback = feedback;
   }
diff --git a/pitest/src/main/java/org/pitest/testapi/execute/containers/UnContainer.java b/pitest/src/main/java/org/pitest/testapi/execute/containers/UnContainer.java
index a1dde0217..8f228be56 100644
--- a/pitest/src/main/java/org/pitest/testapi/execute/containers/UnContainer.java
+++ b/pitest/src/main/java/org/pitest/testapi/execute/containers/UnContainer.java
@@ -16,7 +16,9 @@
 package org.pitest.testapi.execute.containers;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
+import java.util.concurrent.ConcurrentLinkedDeque;
 
 import org.pitest.testapi.TestResult;
 import org.pitest.testapi.TestUnit;
@@ -26,10 +28,10 @@ public class UnContainer implements Container {
 
   @Override
   public List<TestResult> execute(final TestUnit group) {
-    final List<TestResult> results = new ArrayList<>(12);
+    final Collection<TestResult> results = new ConcurrentLinkedDeque<>();
     final ConcreteResultCollector rc = new ConcreteResultCollector(results);
     group.execute(rc);
-    return results;
+    return new ArrayList<>(results);
   }
 
 }