It???s very difficult to buy inaccurate heart rate monitors nowadays. Even the cheapest ones are good in terms of accuracy. As the price increases, so do the number of functions. The very cheap ones give you an updated heart rate every minute. For a couple of bucks more you can get your current heart rate continuously. Personally, I???d buy one that vibrates when you???re outside your target heart rate. The ones that alert you visually are useful only if you constantly look and pay attention. The ones that play sounds to alert you are useful only if it???s not too noisy. Also, it should record the number of calories burned during an exercise. Monitors without a chest strap can be very inaccurate. The best brands out there are Polar and Omega.