The second law specifies conditions under which a cyclically operating body can do positive net work. The conditions are stated in terms of the exchange of heat of the body with its environment. In this respect the second law is similar to the first. The difference is that while for the formulation of the first law the single net gain of heat of the body suffices, the second law requires a much finer concept which distinguishes amounts of heat absorbed or emitted at different temperatures. The concepts of heating measure and accumulation function, to be introduced in Sect. 7.1, serve this purpose. It will be seen that the framework of heating measures enables one to remove the traditional ambiguities in the statements of the second law.