I've had a lot of people tell me they'd like to watch 24, but they haven't seen it from the beginning, and worry they might be lost. The BBC apparently feels their pain, and has this handy guide to everything that happened in the first five seasons.

Tom Coombe

I wholeheartedly agree with the sentiment - I would recommend watching them from the beginning if nothing else to allow some of the moments from later seasons to have full weight. For example, I started watching at season 5, and some of the deaths in the first few episodes really diminished huge moments of suspense in earlier seasons. It's a daunting task, to say the least, but one well worth undertaking - especially if you start watching and get hooked.