diff --git a/app/src/main/java/net/kollnig/missioncontrol/data/TrackerBlocklist.java b/app/src/main/java/net/kollnig/missioncontrol/data/TrackerBlocklist.java
index de90b806..3d47587a 100644
--- a/app/src/main/java/net/kollnig/missioncontrol/data/TrackerBlocklist.java
+++ b/app/src/main/java/net/kollnig/missioncontrol/data/TrackerBlocklist.java
@@ -58,7 +58,8 @@ private TrackerBlocklist(Context c) {
      * @return The current instance of the TrackerBlocklist, if none, a new instance
      *         is created.
      */
-    public static TrackerBlocklist getInstance(Context c) {
+    // Called from both native packet threads and UI threads.
+    public static synchronized TrackerBlocklist getInstance(Context c) {
         if (instance == null)
             instance = new TrackerBlocklist(c);
 
@@ -85,7 +86,7 @@ public static String getBlockingKey(Tracker t) {
      *
      * @param c Context
      */
-    public void loadSettings(Context c) {
+    public synchronized void loadSettings(Context c) {
         SharedPreferences prefs = c.getSharedPreferences(PREF_BLOCKLIST, Context.MODE_PRIVATE);
         Set<String> set = prefs.getStringSet(SHARED_PREFS_BLOCKLIST_APPS_KEY, null);
         PackageUidResolver resolver = new PackageUidResolver() {
@@ -226,14 +227,14 @@ public Set<Integer> getBlocklist() {
      * @param uid Uid of the app
      * @return Information about what specific trackers are blocked
      */
-    public Set<String> getSubset(int uid) {
+    public synchronized Set<String> getSubset(int uid) {
         return blockmap.get(uid);
     }
 
     /**
      * Completely clear blocklist.
      */
-    public void clear() {
+    public synchronized void clear() {
         blockmap.clear();
     }
 
@@ -242,7 +243,7 @@ public void clear() {
      *
      * @param uid Uid of app
      */
-    public void clear(int uid) {
+    public synchronized void clear(int uid) {
         blockmap.remove(uid);
     }
 
@@ -303,7 +304,7 @@ public synchronized void unblock(int uid, Tracker t) {
      * @param key Key of the tracker
      * @return Whether access to this tracker is blocked
      */
-    public boolean blocked(int uid, String key) {
+    public synchronized boolean blocked(int uid, String key) {
         Set<String> trackers = this.getSubset(uid);
         if (trackers == null) {
             return true;
@@ -319,7 +320,7 @@ public boolean blocked(int uid, String key) {
      * @param t   Tracker
      * @return Whether access to this tracker is blocked
      */
-    public boolean blockedTracker(int uid, Tracker t) {
+    public synchronized boolean blockedTracker(int uid, Tracker t) {
         return blocked(uid, t.category)
                 && blocked(uid, getBlockingKey(t));
     }
diff --git a/app/src/test/java/net/kollnig/missioncontrol/data/TrackerBlocklistTest.java b/app/src/test/java/net/kollnig/missioncontrol/data/TrackerBlocklistTest.java
index 08f7295c..4e282bbb 100644
--- a/app/src/test/java/net/kollnig/missioncontrol/data/TrackerBlocklistTest.java
+++ b/app/src/test/java/net/kollnig/missioncontrol/data/TrackerBlocklistTest.java
@@ -21,6 +21,10 @@
 import org.junit.Before;
 import org.junit.Test;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
 public class TrackerBlocklistTest {
     private static final int UID = 1001;
 
@@ -202,4 +206,45 @@ public void blockedTrackerRequiresBothCategoryAndKeyBlocked() {
         blocklist.block(UID, tracker);
         assertTrue(blocklist.blockedTracker(UID, tracker));
     }
+
+    @Test
+    public void concurrentReadersAndWritersDoNotThrow() throws Exception {
+        TrackerBlocklist blocklist = TrackerBlocklist.getInstance(null);
+        int uidCount = 8;
+        for (int uid = 0; uid < uidCount; uid++)
+            blocklist.ensureDefaults(uid, false);
+
+        final int iterations = 1000;
+        List<Throwable> errors = Collections.synchronizedList(new ArrayList<>());
+
+        Runnable writer = () -> {
+            try {
+                for (int i = 0; i < iterations; i++) {
+                    blocklist.applyStrictModeToAll(i % 2 == 0);
+                    blocklist.unblock(0, "Advertising | Writer");
+                    blocklist.block(0, "Advertising | Writer");
+                }
+            } catch (Throwable t) {
+                errors.add(t);
+            }
+        };
+        Runnable reader = () -> {
+            try {
+                for (int i = 0; i < iterations; i++)
+                    for (int uid = 0; uid < uidCount; uid++)
+                        blocklist.blocked(uid, "Content");
+            } catch (Throwable t) {
+                errors.add(t);
+            }
+        };
+
+        Thread[] threads = {new Thread(writer), new Thread(writer),
+                new Thread(reader), new Thread(reader)};
+        for (Thread thread : threads)
+            thread.start();
+        for (Thread thread : threads)
+            thread.join(30000);
+
+        assertTrue(errors.toString(), errors.isEmpty());
+    }
 }