If you don't know what a heart-rate monitor does, here's the lowdown. A chest belt detects your heart rate and transmits it to a wristwatch receiver. It provides an accurate reflection of how hard you're exercising: our own guesses are notoriously unreliable and mean that we can all too easily end up running faster or slower than is good for us.
Most monitors can be set to beep when you stray from your target heart rate, and after your run, some will tell you how long you've spent in your target zone. Those are the basics, and they'll help you to run at your most efficient level for improved fitness.
You don't have to spend much more than £30 for a simple, effective model, but the sky really is the limit if you want to get more complex. Calorie estimators and PC downloads are just the tip of the high-tech iceberg...
How to choose a heart-rate monitor | How to use a heart-rate monitor