Byterun is a Python interpreter implemented in Python. Through my work on Byterun, I was surprised and delighted to discover that the fundamental structure of the Python interpreter fits easily into the 500-line size restriction. This chapter will walk through the structure of the interpreter and give you enough context to explore it further. The goal is not to explain everything there is to know about interpreters—like so many interesting areas of programming and computer science, you could devote years to developing a deep understanding of the topic.