Publisher This chapter discusses three separate aspects of excretion, as first proposed by Maddrell: removal of molecules that are undesirable or perhaps even poisonous at all except very low concentrations; excretion of molecules that are not toxic but merely useless, and that would become obstructive if allowed to accumulate; and excretion of molecules that are useful or essential but are present to excess. This includes excretion of water and physiological ions in some circumstances. The chapter attempts to link recent advances in the understanding of the physiology, biochemistry, genetics, molecular biology, and chemical ecology of insect excretory mechanisms to the fundamental principles of design established several years ago. For example, the primary ATP-dependent transporter in insect epithelia was referred to in the literature until the early 1990s as the electrogenic alkali cation pump. The theme of this chapter is that studies of insect excretory mechanisms frequently provide illustrations of the Krogh principle; for a large number of problems, there will be some animal of choice or a few such animals on which it can be most conveniently studied. Adaptations to extreme environments, coupled with the large number of unusual food sources exploited by insects and their extraordinary diversity, provides a fertile ground for research into excretory mechanisms.