diff --git a/com.google.eclipse.protobuf/META-INF/MANIFEST.MF b/com.google.eclipse.protobuf/META-INF/MANIFEST.MF
index f4159fc..be7f077 100644
--- a/com.google.eclipse.protobuf/META-INF/MANIFEST.MF
+++ b/com.google.eclipse.protobuf/META-INF/MANIFEST.MF
@@ -21,6 +21,7 @@
  org.apache.commons.logging
 Bundle-RequiredExecutionEnvironment: J2SE-1.5
 Export-Package: com.google.eclipse.protobuf,
+ com.google.eclipse.protobuf.grammar,
  com.google.eclipse.protobuf.parseTreeConstruction,
  com.google.eclipse.protobuf.parser.antlr,
  com.google.eclipse.protobuf.parser.antlr.internal,
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
index a14878e..0f0c32e 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
@@ -108,7 +108,9 @@
   | BooleanRef
   | IntRef
   | FloatRef
-  | StringRef;
+  | DoubleRef
+  | StringRef
+  | Nan;
 
 LiteralRef:
   literal=[Literal];
@@ -126,9 +128,15 @@
 FloatRef:
   float=FLOAT;
 
+DoubleRef:
+  double=DOUBLE;
+
 StringRef:
   string=STRING;
 
+Nan:
+  number='nan';
+
 Enum:
   'enum' name=Name '{'
   literals+=Literal*
@@ -163,4 +171,5 @@
 
 terminal INT returns ecore::EInt: ('-')? ('0'..'9')+;  
 terminal FLOAT returns ecore::EFloat: ('-')? ('0'..'9')* ('.' ('0'..'9')+)?;
+terminal DOUBLE returns ecore::EDouble: ('-')? ('0'..'9')* ('.' ('0'..'9')+)?;
   
\ No newline at end of file
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/grammar/CommonKeyword.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/grammar/CommonKeyword.java
new file mode 100644
index 0000000..d00648c
--- /dev/null
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/grammar/CommonKeyword.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2011 Google Inc.
+ *
+ * All rights reserved. This program and the accompanying materials are made available under the terms of the Eclipse
+ * Public License v1.0 which accompanies this distribution, and is available at
+ *
+ * http://www.eclipse.org/legal/epl-v10.html
+ */
+package com.google.eclipse.protobuf.grammar;
+
+/**
+ * A commonly used keyword.
+ *
+ * @author alruiz@google.com (Alex Ruiz)
+ */
+public enum CommonKeyword {
+
+  // we used to get keywords from IGrammarAccess. The problem was that we still had to hardcode the keyword we were
+  // looking for. The code was too complicated and if the grammar changed for some reason, we had to change our
+  // implementation anyway.
+
+  BOOL("bool"), TRUE("true"), FALSE("false"), BYTES("bytes"), OPENING_BRACKET("["), CLOSING_BRACKET("]"),
+    DEFAULT("default"), EQUAL("="), SEMICOLON(";"), STRING("string"), SYNTAX("syntax");
+
+  private final String value;
+
+  private CommonKeyword(String value) {
+    this.value = value;
+  }
+
+  /**
+   * Indicates whether the value of this keyword is equal to the given {@code String}.
+   * @param s the value to compare to.
+   * @return {@code true} if the value of this keyword is equal to the given {@code String}, {@code false} otherwise.
+   */
+  public boolean hasValue(String s) {
+    return value.equals(s);
+  }
+
+  /**
+   * Returns the textual value of this keyword.
+   * @return the textual value of this keyword.
+   */
+  @Override public String toString() {
+    return value;
+  }
+}
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/util/Properties.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/util/Properties.java
new file mode 100644
index 0000000..3ddcd47
--- /dev/null
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/util/Properties.java
@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2011 Google Inc.
+ *
+ * All rights reserved. This program and the accompanying materials are made available under the terms of the Eclipse
+ * Public License v1.0 which accompanies this distribution, and is available at
+ *
+ * http://www.eclipse.org/legal/epl-v10.html
+ */
+package com.google.eclipse.protobuf.util;
+
+import static com.google.eclipse.protobuf.grammar.CommonKeyword.*;
+
+import com.google.eclipse.protobuf.grammar.CommonKeyword;
+import com.google.eclipse.protobuf.protobuf.*;
+import com.google.inject.Singleton;
+
+/**
+ * Utility methods re <code>{@link Property}</code>.
+ *
+ * @author alruiz@google.com (Alex Ruiz)
+ */
+@Singleton
+public class Properties {
+
+  /**
+   * Indicates whether the type of the given property is primitive. Primitive types include: {@code double},
+   * {@code float}, {@code int32}, {@code int64}, {@code uint32}, {@code uint64}, {@code sint32}, {@code sint64},
+   * {@code fixed32}, {@code fixed64}, {@code sfixed32}, {@code sfixed64} and {@code bool}.
+   * @param p the given property.
+   * @return {@code true} if the type of the given property is primitive, {@code false} otherwise.
+   */
+  public boolean isPrimitive(Property p) {
+    AbstractTypeReference r = p.getType();
+    if (!(r instanceof ScalarTypeReference)) return false;
+    String typeName = ((ScalarTypeReference) r).getScalar().getName();
+    return !STRING.hasValue(typeName) && !BYTES.hasValue(typeName);
+  }
+
+  /**
+   * Indicates whether the given property is of type {@code bool}.
+   * @param p the given property.
+   * @return {@code true} if the given property is of type {@code bool}, {@code false} otherwise.
+   */
+  public boolean isBool(Property p) {
+    return isScalarType(p, BOOL);
+  }
+
+  /**
+   * Indicates whether the given property is of type {@code string}.
+   * @param p the given property.
+   * @return {@code true} if the given property is of type {@code string}, {@code false} otherwise.
+   */
+  public boolean isString(Property p) {
+    return isScalarType(p, STRING);
+  }
+
+  private boolean isScalarType(Property p, CommonKeyword typeKeyword) {
+    return typeKeyword.hasValue(typeNameOf(p));
+  }
+
+  /**
+   * Returns the name of the type of the given <code>{@link Property}</code>.
+   * @param p the given {@code Property}.
+   * @return the name of the type of the given {@code Property}.
+   */
+  public String typeNameOf(Property p) {
+    AbstractTypeReference r = p.getType();
+    if (r instanceof ScalarTypeReference) return ((ScalarTypeReference) r).getScalar().getName();
+    if (r instanceof TypeReference) {
+      Type type = ((TypeReference) r).getType();
+      return type == null ? null : type.getName();
+    }
+    return r.toString();
+  }
+}
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java
index cbd6fa0..7bbe1d8 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java
@@ -17,7 +17,9 @@
 
   public static String expectedFieldName;
   public static String expectedFieldNumber;
+  public static String expectedString;
   public static String expectedSyntaxIdentifier;
+  public static String expectedTrueOrFalse;
   public static String fieldNumberAlreadyUsed;
   public static String fieldNumbersMustBePositive;
   public static String importNotFound;
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties
index dbb5a26..cc7c6ed 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties
@@ -1,6 +1,8 @@
 expectedFieldName = Expected field name.
 expectedFieldNumber = Expected field number.
+expectedString = Expected string.
 expectedSyntaxIdentifier = Expected syntax identifier.
+expectedTrueOrFalse = Expected "true" or "false".
 fieldNumberAlreadyUsed = Field number %d has already been used in \"%s\" by field \"%s\".
 fieldNumbersMustBePositive = Field numbers must be positive integers.
 importNotFound = Import \"%s\" was not found.
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java
index 22a6323..a818e82 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java
@@ -8,6 +8,7 @@
  */
 package com.google.eclipse.protobuf.validation;
 
+import static com.google.eclipse.protobuf.protobuf.Modifier.OPTIONAL;
 import static com.google.eclipse.protobuf.protobuf.ProtobufPackage.Literals.*;
 import static com.google.eclipse.protobuf.validation.Messages.*;
 import static java.lang.String.format;
@@ -15,11 +16,13 @@
 
 import org.eclipse.emf.common.util.URI;
 import org.eclipse.emf.ecore.EObject;
-import org.eclipse.xtext.naming.*;
+import org.eclipse.xtext.naming.IQualifiedNameProvider;
+import org.eclipse.xtext.naming.QualifiedName;
 import org.eclipse.xtext.validation.Check;
 
 import com.google.eclipse.protobuf.protobuf.*;
 import com.google.eclipse.protobuf.protobuf.Package;
+import com.google.eclipse.protobuf.util.Properties;
 import com.google.inject.Inject;
 
 /**
@@ -28,6 +31,20 @@
 public class ProtobufJavaValidator extends AbstractProtobufJavaValidator {
 
   @Inject private IQualifiedNameProvider qualifiedNameProvider;
+  @Inject private Properties properties;
+
+  @Check public void checkDefaultValueType(Property property) {
+    ValueRef defaultValue = property.getDefault();
+    if (!OPTIONAL.equals(property.getModifier()) || defaultValue == null) return;
+    if (properties.isString(property)) {
+      if (defaultValue instanceof StringRef) return;
+      error(expectedString, PROPERTY__DEFAULT);
+    }
+    if (properties.isBool(property)) {
+      if (defaultValue instanceof BooleanRef) return;
+      error(expectedTrueOrFalse, PROPERTY__DEFAULT);
+    }
+  }
 
   @Check public void checkImportIsResolved(Import anImport) {
     String importUri = anImport.getImportURI();