The Garmin Forerunner is a GPS device that is worn on the wrist, designed primarily for use by recreational road runners. It is an accurate method of measuring parameters important to runners such as distance, speed, time, altitude, and pace.
Garmin has three models of the Forerunner portable GPS: the 101, the 201, and the 301. With the Garmin Forerunner 201 and Forerunner 301 you can upload run track points to a personal computer, where a you can view a map of your run, see your progression over the course of the run in pace and altitude, and even apply the map of your run over a satellite image of the area where you ran. This is done via a docking station and software that come included with the Forerunner unit.

The smaller but more sensitive Garmin Forerunner 205 and 305 were in 2006. These models use a much more sensitive GPS receiver chip, the SiRF III.

Since the Forerunner is based on GPS technology, in order to function properly it requires a relatively clear view of the sky to the satellites. As a result, if a user wears their Forerunner in an area heavily covered by trees or blockage of the sky, the Forerunner may not record track points. In doing so, a shorter distance may be recorded as the actual distance.

The Forerunner 101 is the only of the three versions to use disposable battery power, all the others use rechargable batteries. Consequently, battery life is limited to about four hours, which may not be sufficient for long-distance runners and marathon runners in particular. The Forerunners 201 and 301, have a battery life of approximately 15 hours and may be a beeter choice for long-distance runners.