What I would add to the others' comments is the way Cathe will throw in a few intervals here and there, or the way she inserts power 15's, 7's, and 3's. These are short, intense cardios with some important interval training frequently thrown in to up the ante, so to speak. I think 10-10-10 is toughest, then the Circuit s, then Step Intervals, then All Step, and finally the Kickbox--but they all get you going!