My approach to situations when there are no unit tests for existing legacy code is to start by writing a few coarse grained, or basic tests. Just enough to get

My approach to situations when there are no unit tests for existing legacy code is to start by writing a few coarse grained, or basic tests. Just enough to get initial coverage on critical functionality. Then you can add a few more tests over time. What also works well is to give someone new to the project some initial tasks to write and fill in gaps for missing unit tests and coverage. Just enough to get them productive from the start, but don't task them with writing all the missing tests. I've found that for most developers that it helps them come up to speed on getting their environment configured, looking at the code, and introduces them to your build and version control process from the start. Ultimately it's going to depend on the project and how hard it is to go back and add the tests.