These last two chapters have been included to illustrate some principles relating to the energy relationships of certain familiar physiological functions, with particular emphasis on the way in which ATP may be used. Although the topics of these two chapters appear to be linked, the manner in which the energy relationships function is very different. Other examples could have been chosen but the number discussed had to be limited; the examples have to some extent been arbitrary, although these two topics are good examples of how biochemists, physiologists and those working on the fine structure of cells have co-operated to provide the type of overall picture that we have today.