Guido's original implementation is a traditional bytecode interpreter written in C, and consequently known as CPython. There are now many other Python implementations. Among the most notable are Jython, which is written in Java and allows for interfacing with Java code, IronPython, which is written in C# and interfaces with Microsoft's .NET framework, and PyPy, the subject of this chapter. CPython is still the most widely used implementation and currently the only one to support Python 3, the next generation of the Python language. This chapter will explain the design decisions in PyPy that make it different from other Python implementations and indeed from any other dynamic language implementation.