Thermostat is usually the most likely thing. Leaky radiator finishes at a strong second. There are plenty of things that can cause it, but those two are the most common. Are you losing coolant?
Bad thermostat - causes coolant to not circulate
Leaky Radiator - causes loss of coolant which can lead to overheating
Leaky hoses - same as above
Bad water pump - same as above plus poor circulation
Bad radiator cap - allows coolant to boil
Radiator fans not functioning - not enough airflow at low speed/idle
Blown headgasket - combustion heat enters water jacket
Insufficient airflow - huge intercooler? bad ducting?
