The world runs on legacy code. For every greenfield progressive web app with 100% test coverage, there are literally hundreds of archaic line-of-business applications running in production - systems with no tests, no documentation, built using out-of-date tools, languages and platforms. It’s the code developers love to hate: it’s not exciting, it’s not shiny, and it won’t look good on your CV - but the world runs on legacy code, and, as developers, if we’re going to work on anything that actually matters, we’re going to end up dealing with legacy. To work effectively with this kind of system, we need to answer some fundamental questions: why was it built this way in the first place? What's happened over the years it's been running in production? And, most importantly, how can we develop our understanding of legacy codebases to the point where we're confident that we can add features, fix bugs and improve performance without making things worse?