Overheating is caused by anything that leads to a loss of coolant, prevents the cooling system from getting rid of heat, or causes excess heat in the engine itself:
Coolant leaks (water pump, radiator, heater core, hoses, freeze plugs, head gasket, engine internal).
Weak radiator cap (does not hold rated pressure and allows coolant to boil over). Pressure test the cap to check it out.
Cooling system clogged (deposits built up in radiator or in engine due to maintenance neglect or use of hard water). Use a cleaner, then reverse flush the system to clean it out. A badly-clogged radiator may need to be rodded out or replaced.
Thermostat stuck shut (replace).
Inoperative electric cooling fan (check fan motor, relay and temperature switch for correct operation).
Bad fan clutch (replace if slipping, leaking or loose).
Missing fan shroud (reduces cooling efficiency of fan).
Slipping fan belt (tighten or replace).
Too low or too high a concentration of antifreeze (should be 50/50 for best cooling).
Bad water pump (impeller eroded or loose – replace pump).
Collapsed radiator hose (check lower hose).
Debris in the radiator (remove bugs and dirt).
Late ignition timing (reset to specs).
Restricted exhaust system (check intake vacuum readings and inspect converter, muffler and pipes).
Radiator and/or fan undersized for application (increase cooling power by installing larger radiator and/or auxiliary cooling fan).