Most of these other answers are explaining the general concept of serialization, but the OP seems to be asking a more specific question: they understand what serialization is, but they've noticed that many codebases serialize Java objects (e.g. to JSON) without implementing the Serializable interface. If this is the case, what's the purpose of the interface? Let me start off by saying that in my experience, the Serializable does not seem to be used very often in modern codebases. Nowadays, most modern codebases tend to prefer to do serialization via a library like Gson, Jackson, SnakeYAML, protobuf, etc. Now to directly answer your question, I think you haven't completely internalized the purpose of an interface yet. Let me do an analogy with electrical outlets. The electrical grid in America vs Europe are built to different standards. In the USA, an electrical outlet will provide 120 volts at 60 Hz, while in many places in Europe, it will provide 240 Volts at 50 Hz. If you want to build an appliance, say a television, you can build one that works to the US standard, or you can build one that works to the European standard. Either one works fine, but you as a TV designer, need to know ahead of time whether you're expecting to receive a US-style power source or a European-style power source so that you can put the right type of circuitry to make sure your television works. Interfaces (in all programming languages, not just Java) act like standards and specifications that allow two software modules to work together, just like the US electrical standard is designed so that any US appliance can work in any US electrical outlet. They provide a guarantee about what sort of functionality an implementing object will provide (e.g. 120 volts at 60Hz) that the caller can rely on to build whatever additional functionality they want on top of it. If you want to integrate with the java.io.ObjectInputStream and java.io.ObjectOutputStream pair of classes (which help perform serialization for you), you need to implement the Serializable interface, just like if you want your television to work with US outlets, you need to design it to work with 120 volts and 60 hz. If you don't care about using ObjectInputStream and ObjectOutputStream, you don't have to implement that interface (maybe your television uses batteries or has a solar panel, in which case you don't need to worry about designing a plug that fits a US outlet). In languages with static type checking, such as Java, the compiler will prevent you from using a class in an API that requires a specific interface is that class does not implement that interface. This is analogous to the idea that the shapes of US electrical outlets are different from European outlets, so you can't accidentally plug in a TV into an outlet that it won't work with. TL;DR: So the answer to your question is that the Serializable is needed when you interact with an API that requires an instance of Serializable -- the vast majority of type, that API will be the java.io.ObjectInputStream and java.io.ObjectOutputStream APIs. If you don't interact with those kinds of APIs, you don't need to implement the Serializable. And in fact, most modern codebases don't rely on ObjectInputStream and ObjectOutputStream to perform serialization, and so most codebases won't bother to have their classes implement Serializable even if though they still intended to serialize those objects -- they simply intend to use a different API to perform that serialization. Answer from Nebu on reddit.com