# Prevost Theory of Exchange

I am having a trouble in understanding this concept. If bodies are at thermal equilibrium, there is no heat transfer. Heat is transferred through three different ways: Conduction, convection and Radiation. Well,according to Prevost theory all bodies radiate thermal radiation at all temperatures. Well, that would include even when they are in thermal equilibrium? If yes, then how so, there shouldn't be any heat transfer. I understand that, if it has to remain in the same temperature the heat radiated out must equal heat radiated in, but I don't understand why in the first place heat is radiated out.