thermostat
coolant level
loose fan belt
clogged coolant passages in engine
bad radiator
clogged/collapsed hose(s)
bad water pump
something wrong with engine - head gasket leak
clogged exhaust system
bad temperature sensor
engine timing is off?
lean carburetor setting?

http://www.aa1car.com/library/overheat.htm

http://www.troubleshooters.com/toverheat.htm

Under what conditions does the light come on?