[tyroh]

1. If the original authors are still around, we do a knowledge transfer. 2. Create diagrams about the relationships between all the systems involved. UMLs, ERDs, and all the rest of them 3. Clone the system locally and trace how the data flows from one point to the next. I'd go with logins first since we all know how it should work so it's easy to follow. 4. Lots and lots of trial and error when contributing new features. If it's well made, tests will catch your bugs. If you have a good team, they'll cut you some slack since you're just new and they were also new, too.