diff --git a/README.org b/README.org
index 918137b..1550087 100644
--- a/README.org
+++ b/README.org
@@ -25,8 +25,10 @@
    - [Maybe]
    - [Result]
    - [Tree]
+   - [Lazy]
    - [Either]
 
+
 ** Maven Usage
 
 #+BEGIN_SRC xml
@@ -925,6 +927,51 @@
      #+END_SRC
 
 
+** Lazy
+
+   A Lazy evaluated expression. Using a Supplier to provide the value, only
+   evaluates the value when required, and never more than once.
+
+*** Static Constructors
+
+**** =static <R> Lazy<R> of(Supplier<R> supplier)=
+
+     Create a new Lazy value from the supplier.
+
+     #+BEGIN_SRC java
+       final Suppler<UUID> supplier = ...;
+       final Lazy<UUID> lazy = Lazy.of(supplier);
+     #+END_SRC
+
+*** Instance Methods
+
+**** =boolean isEvaluated()=
+
+     Checks if the value has been evaluated.
+
+     #+BEGIN_SRC java
+       final Lazy<UUID> lazy = ...;
+       final boolean isEvaluated = lazy.isEvaluated();
+     #+END_SRC
+
+**** =T value()=
+
+     The value, evaluating it if necessary.
+
+     #+BEGIN_SRC java
+       final Lazy<UUID> lazy = ...;
+       final UUID value = lazy.value();
+     #+END_SRC
+
+**** =<R> Lazy<R> map(Function<T, R> f)=
+
+     Maps the Lazy instance into a new Lazy instance using the function.
+
+     #+BEGIN_SRC java
+       final Lazy<UUID> uuidLazy = ...;
+       final Lazy<String> stringLazy = uuidLazy.map(v -> v.toString());
+     #+END_SRC
+
 ** Either
 
    Allows handling a value that can be one of two types, a left value/type or a
diff --git a/src/main/java/net/kemitix/mon/lazy/Lazy.java b/src/main/java/net/kemitix/mon/lazy/Lazy.java
new file mode 100644
index 0000000..ebd593c
--- /dev/null
+++ b/src/main/java/net/kemitix/mon/lazy/Lazy.java
@@ -0,0 +1,67 @@
+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2018 Paul Campbell
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of this software
+ * and associated documentation files (the "Software"), to deal in the Software without restriction,
+ * including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all copies
+ * or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+ * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
+ * AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+ * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package net.kemitix.mon.lazy;
+
+import net.kemitix.mon.Functor;
+
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+/**
+ * Lazy evaluation of 'expensive' expressions.
+ *
+ * @param <T> the type of the value
+ *
+ * @author Paul Campbell (pcampbell@kemitix.net)
+ */
+public interface Lazy<T> extends Functor<T, Lazy<?>> {
+
+    /**
+     * Create a new Lazy value from the supplier.
+     *
+     * @param supplier the source of the value
+     * @param <R> the type of the value
+     * @return a Lazy wrapper of the Supplier
+     */
+    public static <R> Lazy<R> of(final Supplier<R> supplier) {
+        return new LazySupplier<>(supplier);
+    }
+
+    /**
+     * Checks if the value has been evaluated.
+     *
+     * @return true if the value has been evaluated.
+     */
+    public abstract boolean isEvaluated();
+
+    /**
+     * The value, evaluating it if necessary.
+     *
+     * <p>Does not evaluate the value more than once.</p>
+     *
+     * @return the evaluated value
+     */
+    public abstract T value();
+
+    @Override
+    public abstract <R> Lazy<R> map(Function<T, R> f);
+}
diff --git a/src/main/java/net/kemitix/mon/lazy/LazySupplier.java b/src/main/java/net/kemitix/mon/lazy/LazySupplier.java
new file mode 100644
index 0000000..061c61b
--- /dev/null
+++ b/src/main/java/net/kemitix/mon/lazy/LazySupplier.java
@@ -0,0 +1,75 @@
+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2018 Paul Campbell
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of this software
+ * and associated documentation files (the "Software"), to deal in the Software without restriction,
+ * including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all copies
+ * or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+ * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
+ * AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+ * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package net.kemitix.mon.lazy;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+/**
+ * Implementation of Lazy using a Supplier.
+ *
+ * @param <T> the type of the value
+ *
+ * @author Paul Campbell (pcampbell@kemitix.net)
+ */
+class LazySupplier<T> implements Lazy<T> {
+
+    private final Supplier<T> supplier;
+    private final AtomicBoolean evaluated = new AtomicBoolean(false);
+    private final AtomicReference<T> value = new AtomicReference<>();
+
+    /**
+     * Creates a new Lazy wrapper for the Supplier.
+     *
+     * @param supplier the source of the value
+     */
+    LazySupplier(final Supplier<T> supplier) {
+        this.supplier = supplier;
+    }
+
+    @Override
+    public boolean isEvaluated() {
+        return evaluated.get();
+    }
+
+    @Override
+    public T value() {
+        if (evaluated.get()) {
+            return value.get();
+        }
+        synchronized (value) {
+            if (!evaluated.get()) {
+                value.set(supplier.get());
+                evaluated.set(true);
+            }
+        }
+        return value.get();
+    }
+
+    @Override
+    public <R> Lazy<R> map(final Function<T, R> f) {
+        return Lazy.of(() -> f.apply(value()));
+    }
+
+}
diff --git a/src/main/java/net/kemitix/mon/lazy/package-info.java b/src/main/java/net/kemitix/mon/lazy/package-info.java
new file mode 100644
index 0000000..0782b4c
--- /dev/null
+++ b/src/main/java/net/kemitix/mon/lazy/package-info.java
@@ -0,0 +1,28 @@
+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2018 Paul Campbell
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of this software
+ * and associated documentation files (the "Software"), to deal in the Software without restriction,
+ * including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all copies
+ * or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+ * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
+ * AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+ * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+/**
+ * Deferred evaluation.
+ *
+ * @author Paul Campbell (pcampbell@kemitix.net)
+ */
+
+package net.kemitix.mon.lazy;
diff --git a/src/test/java/net/kemitix/mon/lazy/LazySupplierTest.java b/src/test/java/net/kemitix/mon/lazy/LazySupplierTest.java
new file mode 100644
index 0000000..e2ccf5c
--- /dev/null
+++ b/src/test/java/net/kemitix/mon/lazy/LazySupplierTest.java
@@ -0,0 +1,141 @@
+package net.kemitix.mon.lazy;
+
+import org.assertj.core.api.WithAssertions;
+import org.junit.Test;
+
+import java.util.UUID;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Supplier;
+
+public class LazySupplierTest implements WithAssertions {
+
+    @Test
+    public void whenCreateLazyThenSupplierIsNotCalled() {
+        //given
+        final AtomicBoolean supplierCalled = new AtomicBoolean(false);
+        final Supplier<UUID> supplier = () -> {
+            supplierCalled.set(true);
+            return UUID.randomUUID();
+        };
+        //when
+        final Lazy<UUID> lazy = Lazy.of(supplier);
+        //then
+        assertThat(supplierCalled).isFalse();
+    }
+
+    @Test
+    public void whenCreateLazyThenIsEvaluatedIsFalse() {
+        //given
+        final Supplier<UUID> supplier = UUID::randomUUID;
+        //when
+        final Lazy<UUID> lazy = Lazy.of(supplier);
+        //then
+        assertThat(lazy.isEvaluated()).isFalse();
+    }
+
+    @Test
+    public void whenValueThenSupplierIsCalled() {
+        //given
+        final AtomicBoolean supplierCalled = new AtomicBoolean(false);
+        final Supplier<UUID> supplier = () -> {
+            supplierCalled.set(true);
+            return UUID.randomUUID();
+        };
+        final Lazy<UUID> lazy = Lazy.of(supplier);
+        //when
+        lazy.value();
+        //then
+        assertThat(supplierCalled).isTrue();
+    }
+
+    @Test
+    public void whenValueThenValueIsSameAsSupplier() {
+        //given
+        final UUID uuid = UUID.randomUUID();
+        final Supplier<UUID> supplier = () -> uuid;
+        final Lazy<UUID> lazy = Lazy.of(supplier);
+        //when
+        final UUID value = lazy.value();
+        //then
+        assertThat(value).isSameAs(uuid);
+    }
+
+    @Test
+    public void whenValueThenIsEvaluatedIsTrue() {
+        //given
+        final Supplier<UUID> supplier = () -> UUID.randomUUID();
+        final Lazy<UUID> lazy = Lazy.of(supplier);
+        //when
+        lazy.value();
+        //then
+        assertThat(lazy.isEvaluated()).isTrue();
+    }
+
+    @Test
+    public void whenValueCalledTwiceThenSupplierIsNotCalledAgain() {
+        //given
+        final AtomicInteger supplierCalledCounter = new AtomicInteger(0);
+        final Supplier<UUID> supplier = () -> {
+            supplierCalledCounter.incrementAndGet();
+            return UUID.randomUUID();
+        };
+        final Lazy<UUID> lazy = Lazy.of(supplier);
+        //when
+        lazy.value();
+        lazy.value();
+        //then
+        assertThat(supplierCalledCounter).hasValue(1);
+    }
+
+    @Test
+    public void whenMapLazyThenSupplierNotCalled() {
+        //given
+        final UUID uuid = UUID.randomUUID();
+        final AtomicBoolean supplierCalled = new AtomicBoolean(false);
+        final Supplier<UUID> supplier = () -> {
+            supplierCalled.set(true);
+            return uuid;
+        };
+        final Lazy<UUID> uuidLazy = Lazy.of(supplier);
+        //when
+        uuidLazy.map(UUID::toString);
+        //then
+        assertThat(supplierCalled).isFalse();
+    }
+
+    @Test
+    public void whenMapLazyValueThenSupplierIsCalled() {
+        //given
+        final UUID uuid = UUID.randomUUID();
+        final AtomicBoolean supplierCalled = new AtomicBoolean(false);
+        final Supplier<UUID> supplier = () -> {
+            supplierCalled.set(true);
+            return uuid;
+        };
+        final Lazy<UUID> uuidLazy = Lazy.of(supplier);
+        final Lazy<String> stringLazy = uuidLazy.map(UUID::toString);
+        //when
+        stringLazy.value();
+        //then
+        assertThat(supplierCalled).isTrue();
+    }
+
+    @Test
+    public void whenMapLazyValueThenValueIsCorrect() {
+        //given
+        final UUID uuid = UUID.randomUUID();
+        final AtomicBoolean supplierCalled = new AtomicBoolean(false);
+        final Supplier<UUID> supplier = () -> {
+            supplierCalled.set(true);
+            return uuid;
+        };
+        final Lazy<UUID> uuidLazy = Lazy.of(supplier);
+        final Lazy<String> stringLazy = uuidLazy.map(UUID::toString);
+        //when
+        final String value = stringLazy.value();
+        //then
+        assertThat(value).isEqualTo(uuid.toString());
+    }
+
+}
\ No newline at end of file