The efficiency of a Carnot engine is treated for the case where the power output is limited by the rates of heat transfer to and from the working substance. It is shown that the efficiency, η, at maximum power output is given by the expression η = 1 − (T2/T1)1/2where T1and T2are the respective temperatures of the heat source and heat sink. It is also shown that the efficiency of existing engines is well described by the above result.