A heart rate monitor gives you an accurate measurement of how hard your body is working. This allows you to adjust how hard you exercise and achieve your fitness goals, it can also let you know if you should not train as you may need more recovery time. All models will display a basic 'rev counter' indicating your current heart rate in beats per minute. More complex HRM's add simultaneous displays of how hard you are working as a percentage of your maximum rate or how long you've managed to maintain your heart rate inside pre-set target zones. This is a great feature for targeting specific training zones, these are often reffered to as the following: The Energy Efficient or Recovery Zone which is 60% to 70%, The Aerobic Zone - 70% to 80%, The Anaerobic Zone - 80% to 90%, The Anaerobic Zone - 80% to 90% of your maximum heart rate. Each zone has a specific benefit and a HRM will help you to stay within these zones and achieve those benefits. The ability to download data directly to a PC for later analysis is also a nice feature if you're following a long-term fitness program, this can also show what's working well for you.