The purpose of this paper is to review the various approaches to second law analysis and to present a rational method which satisfies the physical requirements. It is not the intention to review all previous work, but to present an approach that resolves some perceived inconsistencies and paradoxes. Some new relationships are derived, particularly for the local rate generation process, and for the nearly-balanced counterflow arrangement with a 'long' duty. It is also shown that the basic entropy generation relationship for gas flows is controlled by the flow Mach number, which is consistent with an extension of Shapiro's classical one-dimensional flow analysis of a compressible gas with friction and heat addition.