In this chapter, we learn about the top-down design (TDD) methodology for organizing code in large programming projects. Then we learn how to implement that methodology using Python functions. Implemented thus, they act as subroutines and the main routine of a program. Also implemented thus, we can put our subroutines in one text file (module) and invoke them from the main routine in another text file (module). Organizing our code in this way emphasizes its modularity.