I always do my cardio after my workouts. You want to save you energy for your weight training, so that you can push yourself as hard as possible in your workout. Then after your workout do your cardio. It is one if the prime times to do it. After your workout you will have used up most if not all of your glycogen, so that when you get on the cardio after the workout, you will go right into a fat burning mode, or very shortly after you will be into a fat burning state. I know when I am doing cardio, I want to be burning fat, not just burning the calories from the foods I just ate. You want to have as much energy for your workout as possible, and doing cardio first, for most people zaps alot of that energy you could be using to get a great muscle building workout. Another prime time to do cardio... Actually the best time to do it, is first thing in the morning, right after you get up, before eating anything. You will be on a fast from not eating for the time you were sleeping, so again, you will go into a fat burning state right away. This is a general rule for people trying to lean out/stripping body fat. For those of us trying to hold on to as much muscle while getting ripped up, you can take some BCAA's before the cardio along with your fat burner, if you are taking one. The BCAA's (Branched Chain Amino Acids) will give fuel to your muscles to help spare any muscle wasting during this cardio. Good luck, and I hope this helps you.