This chapter discusses physiological adaptation in the organism. Organisms are treated as machines, energy transformers that are designed to work in a certain way and at certain levels of efficiency under particular ecological conditions. The concept of design is central to biological explanation and within this framework, this chapter discusses two general issues: the design criteria used in the evolution of metabolic machines and the various metabolic strategies important in the operation of these machines. The chapter discusses the way organisms work as energy transformers and the circumstances under which they transform energy and the reason they work in the way they do. In the first instance, one is concerned with the operation of organisms in ecosystems and only in a subsidiary way with the operation of organisms as part of ecosystem metabolism. The latter is, of course, the more usual problem and discusses “ecological energetic”.