diff --git a/pom.xml b/pom.xml
index e58c56b..b8d6cc4 100644
--- a/pom.xml
+++ b/pom.xml
@@ -76,6 +76,10 @@
             <artifactId>junit</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.hamcrest</groupId>
+            <artifactId>hamcrest-library</artifactId>
+        </dependency>
         <dependency>
             <groupId>xmlunit</groupId>
             <artifactId>xmlunit</artifactId>
diff --git a/src/main/java/com/rometools/opml/io/impl/OPML20Generator.java b/src/main/java/com/rometools/opml/io/impl/OPML20Generator.java
index dfc4137..96ff4ae 100644
--- a/src/main/java/com/rometools/opml/io/impl/OPML20Generator.java
+++ b/src/main/java/com/rometools/opml/io/impl/OPML20Generator.java
@@ -1,5 +1,7 @@
 package com.rometools.opml.io.impl;
 
+import java.util.Collection;
+import java.util.List;
 import java.util.Locale;
 
 import org.jdom2.Document;
@@ -12,7 +14,9 @@ import com.rometools.rome.io.FeedException;
 import com.rometools.rome.io.impl.DateParser;
 
 /**
- * @author cooper
+ * Generator for OPML 2.0 documents.
+ *
+ * @see <a href="http://dev.opml.org/spec2.html">http://dev.opml.org/spec2.html</a>
  */
 public class OPML20Generator extends OPML10Generator {
 
@@ -41,33 +45,61 @@ public class OPML20Generator extends OPML10Generator {
      */
     @Override
     public Document generate(final WireFeed feed) throws IllegalArgumentException, FeedException {
-        final Document retValue = super.generate(feed);
-        retValue.getRootElement().setAttribute("version", "2.0");
-        return retValue;
+        final Document document = super.generate(feed);
+        document.getRootElement().setAttribute("version", "2.0");
+        return document;
     }
 
     @Override
     protected Element generateHead(final Opml opml) {
 
-        final Element docs = new Element("docs");
-        docs.setText(opml.getDocs());
+        final Element docsElement = new Element("docs");
+        docsElement.setText(opml.getDocs());
 
-        final Element retValue = super.generateHead(opml);
-        retValue.addContent(docs);
-        return retValue;
+        final Element headElement = super.generateHead(opml);
+        headElement.addContent(docsElement);
+        return headElement;
 
     }
 
     @Override
     protected Element generateOutline(final Outline outline) {
 
-        final Element retValue = super.generateOutline(outline);
+        final Element outlineElement = super.generateOutline(outline);
 
         if (outline.getCreated() != null) {
-            retValue.setAttribute("created", DateParser.formatRFC822(outline.getCreated(), Locale.US));
+            outlineElement.setAttribute("created", DateParser.formatRFC822(outline.getCreated(), Locale.US));
         }
 
-        return retValue;
+        final List<String> categories = outline.getCategories();
+        final String categoryValue = generateCategoryValue(categories);
+        addNotNullAttribute(outlineElement, "category", categoryValue);
+
+        return outlineElement;
+
+    }
+
+    private String generateCategoryValue(final Collection<String> categories) {
+
+        final StringBuilder builder = new StringBuilder();
+
+        boolean first = true;
+        for (final String category : categories) {
+            if (category != null && !category.trim().isEmpty()) {
+                if (first) {
+                    first = false;
+                } else {
+                    builder.append(",");
+                }
+                builder.append(category.trim());
+            }
+        }
+
+        if (builder.length() > 0) {
+            return builder.toString();
+        } else {
+            return null;
+        }
 
     }
 
diff --git a/src/test/java/com/rometools/opml/io/impl/OPML20GeneratorTest.java b/src/test/java/com/rometools/opml/io/impl/OPML20GeneratorTest.java
new file mode 100644
index 0000000..28dd3d1
--- /dev/null
+++ b/src/test/java/com/rometools/opml/io/impl/OPML20GeneratorTest.java
@@ -0,0 +1,73 @@
+package com.rometools.opml.io.impl;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
+
+import java.util.Arrays;
+
+import org.custommonkey.xmlunit.XMLUnit;
+import org.junit.Test;
+import org.w3c.dom.Document;
+import org.w3c.dom.NodeList;
+
+import com.rometools.opml.feed.opml.Opml;
+import com.rometools.opml.feed.opml.Outline;
+import com.rometools.rome.io.WireFeedOutput;
+
+public class OPML20GeneratorTest {
+
+    @Test
+    public void testOutputOfNullCategory() {
+        assertThat(categoryOf((String) null).getLength(), is(equalTo(0)));
+    }
+
+    @Test
+    public void testOutputOfEmptyCategory() {
+        assertThat(categoryOf("").getLength(), is(equalTo(0)));
+    }
+
+    @Test
+    public void testOutputOfBlankCategory() {
+        assertThat(categoryOf(" ").getLength(), is(equalTo(0)));
+    }
+
+    @Test
+    public void testOutputOfOneCategory() {
+        assertThat(categoryValueOf("category1"), is(equalTo("category1")));
+    }
+
+    @Test
+    public void testOutputOfMultipleCategories() {
+        assertThat(categoryValueOf("category1", "category2"), is(equalTo("category1,category2")));
+    }
+
+    private NodeList categoryOf(final String... categories) {
+
+        try {
+
+            final Outline outline = new Outline("outline1", null);
+            outline.setCategories(Arrays.asList(categories));
+
+            final Opml opml = new Opml();
+            opml.setFeedType("opml_2.0");
+            opml.setTitle("title");
+            opml.setOutlines(Arrays.asList(outline));
+
+            final WireFeedOutput output = new WireFeedOutput();
+            final String xml = output.outputString(opml);
+
+            final Document document = XMLUnit.buildControlDocument(xml);
+            return XMLUnit.newXpathEngine().getMatchingNodes("/opml/body/outline/@category", document);
+
+        } catch (final Exception e) {
+            throw new RuntimeException(e);
+        }
+
+    }
+
+    private String categoryValueOf(final String... categories) {
+        return categoryOf(categories).item(0).getNodeValue();
+    }
+
+}