diff --git a/app/build.gradle b/app/build.gradle
index 6bdb20d23..f15b72924 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -77,6 +77,7 @@ dependencies {
     compile 'com.opencsv:opencsv:3.7'
     compile 'org.apmem.tools:layouts:1.10@aar'
     compile 'org.jetbrains:annotations-java5:15.0'
+    compile 'com.google.code.findbugs:jsr305:2.0.1'
 
     provided 'javax.annotation:jsr250-api:1.0'
 
diff --git a/app/src/main/java/org/isoron/uhabits/models/Checkmark.java b/app/src/main/java/org/isoron/uhabits/models/Checkmark.java
index 3ae1c4b1e..617a09db4 100644
--- a/app/src/main/java/org/isoron/uhabits/models/Checkmark.java
+++ b/app/src/main/java/org/isoron/uhabits/models/Checkmark.java
@@ -21,6 +21,8 @@ package org.isoron.uhabits.models;
 
 import org.apache.commons.lang3.builder.*;
 
+import javax.annotation.concurrent.*;
+
 /**
  * A Checkmark represents the completion status of the habit for a given day.
  * <p>
@@ -30,6 +32,7 @@ import org.apache.commons.lang3.builder.*;
  * <p>
  * Checkmarks are computed automatically from the list of repetitions.
  */
+@ThreadSafe
 public final class Checkmark
 {
     /**
diff --git a/app/src/main/java/org/isoron/uhabits/models/CheckmarkList.java b/app/src/main/java/org/isoron/uhabits/models/CheckmarkList.java
index 86488a8d1..2f97d7fd3 100644
--- a/app/src/main/java/org/isoron/uhabits/models/CheckmarkList.java
+++ b/app/src/main/java/org/isoron/uhabits/models/CheckmarkList.java
@@ -27,18 +27,22 @@ import java.io.*;
 import java.text.*;
 import java.util.*;
 
+import javax.annotation.concurrent.*;
+
 /**
  * The collection of {@link Checkmark}s belonging to a habit.
  */
+@ThreadSafe
 public abstract class CheckmarkList
 {
-    protected Habit habit;
+    protected final Habit habit;
 
-    public ModelObservable observable = new ModelObservable();
+    public final ModelObservable observable;
 
     public CheckmarkList(Habit habit)
     {
         this.habit = habit;
+        this.observable = new ModelObservable();
     }
 
     /**
@@ -64,7 +68,7 @@ public abstract class CheckmarkList
      * @return values for the checkmarks in the interval
      */
     @NonNull
-    public final int[] getAllValues()
+    public synchronized final int[] getAllValues()
     {
         Repetition oldestRep = habit.getRepetitions().getOldest();
         if (oldestRep == null) return new int[0];
@@ -97,7 +101,7 @@ public abstract class CheckmarkList
      * @return checkmark for today
      */
     @Nullable
-    public final Checkmark getToday()
+    public synchronized final Checkmark getToday()
     {
         computeAll();
         return getNewestComputed();
@@ -108,7 +112,7 @@ public abstract class CheckmarkList
      *
      * @return value of today's checkmark
      */
-    public final int getTodayValue()
+    public synchronized final int getTodayValue()
     {
         Checkmark today = getToday();
         if (today != null) return today.getValue();
@@ -159,9 +163,14 @@ public abstract class CheckmarkList
      */
     public final void writeCSV(Writer out) throws IOException
     {
-        computeAll();
+        int values[];
+
+        synchronized (this)
+        {
+            computeAll();
+            values = getAllValues();
+        }
 
-        int values[] = getAllValues();
         long timestamp = DateUtils.getStartOfToday();
         SimpleDateFormat dateFormat = DateFormats.getCSVDateFormat();
 
@@ -271,7 +280,7 @@ public abstract class CheckmarkList
      * repetition of the habit until today. Days that already have a
      * corresponding checkmark are skipped.
      */
-    protected final void computeAll()
+    private synchronized void computeAll()
     {
         Repetition oldest = habit.getRepetitions().getOldest();
         if (oldest == null) return;
diff --git a/app/src/main/java/org/isoron/uhabits/models/Frequency.java b/app/src/main/java/org/isoron/uhabits/models/Frequency.java
index 5b893b5a1..b21349b18 100644
--- a/app/src/main/java/org/isoron/uhabits/models/Frequency.java
+++ b/app/src/main/java/org/isoron/uhabits/models/Frequency.java
@@ -21,9 +21,12 @@ package org.isoron.uhabits.models;
 
 import org.apache.commons.lang3.builder.*;
 
+import javax.annotation.concurrent.*;
+
 /**
  * Represents how often is the habit repeated.
  */
+@ThreadSafe
 public class Frequency
 {
     public static final Frequency DAILY = new Frequency(1, 1);
diff --git a/app/src/main/java/org/isoron/uhabits/models/Habit.java b/app/src/main/java/org/isoron/uhabits/models/Habit.java
index 5ba711407..ea0df6f7b 100644
--- a/app/src/main/java/org/isoron/uhabits/models/Habit.java
+++ b/app/src/main/java/org/isoron/uhabits/models/Habit.java
@@ -26,11 +26,13 @@ import org.apache.commons.lang3.builder.*;
 
 import java.util.*;
 
+import javax.annotation.concurrent.*;
 import javax.inject.*;
 
 /**
  * The thing that the user wants to track.
  */
+@ThreadSafe
 public class Habit
 {
     public static final String HABIT_URI_FORMAT =
@@ -54,22 +56,23 @@ public class Habit
     @NonNull
     private boolean archived;
 
-    @NonNull
-    private StreakList streaks;
+    @Nullable
+    private Reminder reminder;
 
     @NonNull
-    private ScoreList scores;
+    private final StreakList streaks;
 
     @NonNull
-    private RepetitionList repetitions;
+    private final ScoreList scores;
 
     @NonNull
-    private CheckmarkList checkmarks;
+    private final RepetitionList repetitions;
 
-    @Nullable
-    private Reminder reminder;
+    @NonNull
+    private final CheckmarkList checkmarks;
 
-    private ModelObservable observable = new ModelObservable();
+    @NonNull
+    private final ModelObservable observable;
 
     /**
      * Constructs a habit with default attributes.
@@ -88,12 +91,13 @@ public class Habit
         streaks = factory.buildStreakList(this);
         scores = factory.buildScoreList(this);
         repetitions = factory.buildRepetitionList(this);
+        observable = new ModelObservable();
     }
 
     /**
      * Clears the reminder for a habit.
      */
-    public void clearReminder()
+    public synchronized void clearReminder()
     {
         reminder = null;
         observable.notifyListeners();
@@ -104,7 +108,7 @@ public class Habit
      *
      * @param model the model whose attributes should be copied from
      */
-    public void copyFrom(@NonNull Habit model)
+    public synchronized void copyFrom(@NonNull Habit model)
     {
         this.name = model.getName();
         this.description = model.getDescription();
@@ -119,7 +123,7 @@ public class Habit
      * List of checkmarks belonging to this habit.
      */
     @NonNull
-    public CheckmarkList getCheckmarks()
+    public synchronized CheckmarkList getCheckmarks()
     {
         return checkmarks;
     }
@@ -133,56 +137,56 @@ public class Habit
      * habit.color).
      */
     @NonNull
-    public Integer getColor()
+    public synchronized Integer getColor()
     {
         return color;
     }
 
-    public void setColor(@NonNull Integer color)
+    public synchronized void setColor(@NonNull Integer color)
     {
         this.color = color;
     }
 
     @NonNull
-    public String getDescription()
+    public synchronized String getDescription()
     {
         return description;
     }
 
-    public void setDescription(@NonNull String description)
+    public synchronized void setDescription(@NonNull String description)
     {
         this.description = description;
     }
 
     @NonNull
-    public Frequency getFrequency()
+    public synchronized Frequency getFrequency()
     {
         return frequency;
     }
 
-    public void setFrequency(@NonNull Frequency frequency)
+    public synchronized void setFrequency(@NonNull Frequency frequency)
     {
         this.frequency = frequency;
     }
 
     @Nullable
-    public Long getId()
+    public synchronized Long getId()
     {
         return id;
     }
 
-    public void setId(@Nullable Long id)
+    public synchronized void setId(@Nullable Long id)
     {
         this.id = id;
     }
 
     @NonNull
-    public String getName()
+    public synchronized String getName()
     {
         return name;
     }
 
-    public void setName(@NonNull String name)
+    public synchronized void setName(@NonNull String name)
     {
         this.name = name;
     }
@@ -203,13 +207,13 @@ public class Habit
      * @throws IllegalStateException if habit has no reminder
      */
     @NonNull
-    public Reminder getReminder()
+    public synchronized Reminder getReminder()
     {
         if (reminder == null) throw new IllegalStateException();
         return reminder;
     }
 
-    public void setReminder(@Nullable Reminder reminder)
+    public synchronized void setReminder(@Nullable Reminder reminder)
     {
         this.reminder = reminder;
     }
@@ -248,17 +252,17 @@ public class Habit
      *
      * @return true if habit has reminder, false otherwise
      */
-    public boolean hasReminder()
+    public synchronized boolean hasReminder()
     {
         return reminder != null;
     }
 
-    public boolean isArchived()
+    public synchronized boolean isArchived()
     {
         return archived;
     }
 
-    public void setArchived(boolean archived)
+    public synchronized void setArchived(boolean archived)
     {
         this.archived = archived;
     }
diff --git a/app/src/main/java/org/isoron/uhabits/models/HabitList.java b/app/src/main/java/org/isoron/uhabits/models/HabitList.java
index 444a5824c..2892a2c30 100644
--- a/app/src/main/java/org/isoron/uhabits/models/HabitList.java
+++ b/app/src/main/java/org/isoron/uhabits/models/HabitList.java
@@ -28,12 +28,15 @@ import org.isoron.uhabits.utils.*;
 import java.io.*;
 import java.util.*;
 
+import javax.annotation.concurrent.*;
+
 /**
  * An ordered collection of {@link Habit}s.
  */
+@ThreadSafe
 public abstract class HabitList implements Iterable<Habit>
 {
-    private ModelObservable observable;
+    private final ModelObservable observable;
 
     @NonNull
     protected final HabitMatcher filter;
diff --git a/app/src/main/java/org/isoron/uhabits/models/ModelObservable.java b/app/src/main/java/org/isoron/uhabits/models/ModelObservable.java
index a762b5f8a..6ab80813b 100644
--- a/app/src/main/java/org/isoron/uhabits/models/ModelObservable.java
+++ b/app/src/main/java/org/isoron/uhabits/models/ModelObservable.java
@@ -21,10 +21,13 @@ package org.isoron.uhabits.models;
 
 import java.util.*;
 
+import javax.annotation.concurrent.*;
+
 /**
  * A ModelObservable allows objects to subscribe themselves to it and receive
  * notifications whenever the model is changed.
  */
+@ThreadSafe
 public class ModelObservable
 {
     private List<Listener> listeners;
@@ -43,7 +46,7 @@ public class ModelObservable
      *
      * @param l the listener to be added.
      */
-    public void addListener(Listener l)
+    public synchronized void addListener(Listener l)
     {
         listeners.add(l);
     }
@@ -53,7 +56,7 @@ public class ModelObservable
      * <p>
      * Only models should call this method.
      */
-    public void notifyListeners()
+    public synchronized void notifyListeners()
     {
         for (Listener l : listeners) l.onModelChange();
     }
@@ -66,7 +69,7 @@ public class ModelObservable
      *
      * @param l the listener to be removed
      */
-    public void removeListener(Listener l)
+    public synchronized void removeListener(Listener l)
     {
         listeners.remove(l);
     }
diff --git a/app/src/main/java/org/isoron/uhabits/tasks/AndroidTaskRunner.java b/app/src/main/java/org/isoron/uhabits/tasks/AndroidTaskRunner.java
index e1626fe34..fac5849f4 100644
--- a/app/src/main/java/org/isoron/uhabits/tasks/AndroidTaskRunner.java
+++ b/app/src/main/java/org/isoron/uhabits/tasks/AndroidTaskRunner.java
@@ -60,7 +60,7 @@ public class AndroidTaskRunner implements TaskRunner
     public void execute(Task task)
     {
         task.onAttached(this);
-        new CustomAsyncTask(task).execute();
+        new CustomAsyncTask(task).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     @Override