Thermostat stuck closed, vanes wore off water pump impeller or the impeller is cracked and loose, cooling fins rotting off the radiator, leaking cylinder head gasket, plugged radiator, blocked radiator air flow, low coolant level, radiator fan not turning on.

Your mechanic will look for clues like how long it takes to overheat, whether it occurs at low speeds or on the highway, whether running the heater reduces engine temperature, if there's hot and cold spots across the radiator, etc.