I've been in a similar situation in the past. The approach I took was to work gradually and try to clean the most heavily-used mess bit-by-bit, under the radar

I've been in a similar situation in the past. The approach I took was to work gradually and try to clean the most heavily-used mess bit-by-bit, under the radar if necessary. For instance, take the particular long, messy function that you happen to be working with right now, then write a bunch of tests for that function. Test that invariants are maintained, bad arguments or pre-conditions are handled, post-conditions are honoured and so on. You don't even have to know whether what you are testing is correct behaviour at this point. Just get as much test coverage as you practically are able to. Then you can start TDDing smaller functions that are made up of bits and pieces from the big, ugly beast-function. You can develop and test these in complete isolation from the running code. Finally you start replacing bits of the ugly beast with calls to your new clean functions, ensuring that you don't break any of those tests you wrote at the start. Over a period of time, you will gradually build up a set of unit tests for the most important parts of the system and you will hopefully gain enough confidence to refactor it into something more maintainable. At least, that's the theory, anyway. :) I would also strongly recommend reading Michael Feathers' excellent book, Working Effectively With Legacy Code (ISBN: 978-0-13-117705-5). It is the bible for this kind of work.