Intervals are simply periods of effort followed by periods of rest, so there is lot of different ways that you could do in an interval work out. You could do with cardiovascular training for example going from walking to running, walking to running or walking to sprinting, walking to sprinting anything more that intensity is going up and coming down. You could do with strength training movements, so then you are doing bicep curls, and then you are taking a breather that's the form of an interval. When you come to type of intervals you choose, first of all depends on your goals but also your fitness levels. For example if you are a beginner, then you probably want recovery times actually be longer than your working times. So for example a beginner runner would they may be able to walk for two minutes, run for a minute walk for two minutes run for a minute. As they get stronger, they could run for longer, walk for shorter period of times and eventually take out all the walking until they are finally able to do full steady straight running. High intensity interval training is where you are taking your intensity very high and then bringing it back down and again just considering the person who is doing these exercises. If you don't feel fully recovered then you may take more recovery time. It is important that you bring your heart rate down between the intervals. And as you get more and more fit than you take less and less time that it takes to recover. That's the sign that your heart is getting stronger. So, when it comes to doing interval training just make sure you are think about intensity that you are working and the amount of time that you are working. And remember beginners, you want your recovery times to be quite longer than the intensity times as you get more conditioned your recovery times can be shorter as your intensity becomes greater.