i do intervals on the step mill but i can also be done on an eliptical or bike. you do an all out sprint as hard as you can go for 30-60 seconds and then you do 2 minutes at a medium pace to recover. you do these blocks for anywhere between 9-15-18 mins and you are done. great for building your endurance up really quickly stripping off fat and not destroying a ton of muscle. good stuff to me across the board