In scientific computing, we sometimes begin with a clean slate; we're given a new problem to solve, and we write software to accomplish the task. In this situation, we?re the inventors. Other times, we work on a problem for which considerable software development has been done, often over a period of many years. The existing software might have had many authors, some of whom have moved on to other positions. In this situation, our job is more akin to detective work. We study the existing code, run examples to see how it behaves, and come to understand both what it does and how it does it.