A good book, though somewhat misleading in its title. Although the exercises contained within are all interesting and helpful, the book is NOT about doing Sit Ups, but building up your core and surrounding areas by means of various exercises that include some situps. The book has definitely strengthened my back, abs, and core, but it lacks focus compared to another book in this series "100 Pushups". There are too many different exercises scattered about in the program - I would have liked to see it more centered around Sit Ups with maybe a couple of ancillary exercises.