If an object is Serializable, and when it is serialized, ojbect's meta data along with its content are written to persistent area. If an object is externalizable, Only it's indentity or meta data is written to persistent area. The class (or object) itself is responsible for writting and reading the content of its instances. http://java.sun.com/j2se/1.4.2/docs/api/java/io/Externalizable.html