Many HRMs use ANT+, a wireless technology that's been around since before Bluetooth became popular and is common in a lot of other sports equipment, from bicycle cadence meters to treadmills. Phones don't typically support ANT+, however. If you want to pair an ANT+ device with a phone and app, you often need an adapter. Heart rate monitors that use Bluetooth are much easier to pair directly with your phone. The most versatile trackers support both ANT+ and Bluetooth.