The importance of using the second law of thermodynamics in the design of heat exchangers, heat exchanger networks, and processes in general, is discussed. The optimal Delta T at a refrigerated heat exchanger is considered from a second law viewpoint. It is shown that the use of minimum total annualized cost as the single optimizing factor is unsatisfactory. Total annualized costs are based on predicted costs of fuel, equipment, and capital, which are uncertain at best. Instead of a singular or ＇＇global optimum＇＇ Delta T, there is a range of optimal Delta Ts over which the total annualized cost is essentially the same, but within which the distribution between cost of capital and cost of energy is significantly different. In selecting a design Delta T, this distribution of costs should also be considered. The possibility of only one singular, or global optimum, solution for complex processes is also considered from a philosophical viewpoint and is again rejected. The existence and identification of design decisions which unnecessarily waste thermodynamic availability (physical exergy) are discussed and identified as ＇＇second law errors.＇＇ Elimination of a second law error from a design guarantees an improved design. An optimal design, which may be any one of a numerous set of optimal designs, will result when all second law errors are eliminated. A design procedure to develop optimal process designs, using such thermodynamic insights, is proposed.