Middle distance track events are track races longer than sprints up to (and arguably including) 5000 meters. Biological factors that go primarily into a middle distance racer are developed fast and type 1 slow twitch muscles, adrenaline (usually for the final kick), anaerobic respiration (bringing it in after the final kick), racing tactics, and aerobic conditioning. The 1500 m has been estimated to require an equal contribution from fast and slow twitch muscles.

Common distances (and not so common ones):
The standard middle distances are the 800 m, 1500 m, mile (in the US), and the 3000 m. The 3000 m is more common in the US at the high school and collegiate level (along with the US two mile) and is in fact not run at the world championships.

600 m
This middle distance length is rather uncommon. The record at this distance for men:
Johnny Gray 1:12.81 Santa Monica, USA 24 May 1986
For women:

Ana Fidelia Quirot 1:22.63 Guadalajara, ESP 25 July 1997

800 m
The 800 m is run over two laps of the track and has always been an Olympic event. It was included in the first women's track programme in 1928, but suspended until 1960 because of shock at the exhaustion it caused the competitors. By contrast, without today's training regimes, male competitors of the day were expected to run themselves to exhaustion in competition.

Tactics