This is one of top serialization questions that is asked in many big companies to test your in-depth understanding of serialization. Serializable is a marker interface therefore you are not forced to implement any methods, however Externalizable contains two methods readExternal() and writeExternal() which must be implemented. Serializable interface provides a inbuilt serialization mechanism to you which can be in-efficient at times. However Externilizable interface is designed to give you greater control over the serialization mechanism. The two methods provide you immense opportunity to enhance the performance of specific object serialization based on application needs. Serializable interface provides a default serialization mechanism, on the other hand, Externalizable interface instead of relying on default Java Serialization provides flexibility to control this mechanism. One can drastically improve the application performance by implementing the Externalizable interface correctly. However there is also a chance that you may not write the best implementation, so if you are not really sure about the best way to serialize, I would suggest your stick to the default implementation using Serializable interface.