Track and field events are divided into three broad categories: track events, field events, and combined events. The majority of athletes tend to specialize in just one event (or event type) with the aim of perfecting their performances, although the aim of combined events athletes is to become proficient in a number of disciplines. Track events involve running on a track over specified distances, and—in the case of the hurdling and steeplechase events—obstacles may be placed on the track. There are also relay races in which teams of athletes run and pass on a baton to their team members at the end of a certain distance.