Using a combination of first-principles calculations and an extended Hückel tight binding model this letter reports on the origin of the low Gilbert damping in half metals. This approach enables the prediction of the lower limit for the magnetization relaxation in a wide variety of material systems relevant for future spintronic applications. For the two model systems Co2MnGe and Co2MnSi minimal damping parameters of 1.9x10−4 and 0.6x10−4 are predicted.