The coolant reservoir is a container that holds the excess or overflowing cooling agent which is used in the vehicles cooling system. It is usually a clear plastic bottle located near the radiator. You can easily find it by following the pipe of the radiator, which will be visible when you open the hood. The coolant reservoir is usually pressurized in newer vehicle models. It is attached to the radiator and the engine with hoses and is a central component in the system. The reservoir itself is the only place where you can actually fill coolant, instead of directly into the radiator. Coolant reservoirs are required because engines go through cycles of expelling and absorbing coolant as they warm up and cool down.