The need for better and more systematic descriptions of the chaetotaxy (especially data concerning the shape, structure and pattern of distribution of the setae) is emphasized. The historical developments of studies in chaetotaxy are reviewed. Two basic types of cuticular processes can be recognized: setae and pseudochaetae. The former have sensorial and mechanical functions, the latter only a mechanical function. A special type of seta is the aesthetasc or the chemosensorial receptor. Using the shape and structure of the setae, most of them can be classified in the following categories: simple, plumed, serrate and chelate. The importance of developmental studies for the establishment of homologies in chaetotaxy is stressed. Examples of functional morphology of setae are discussed. It is emphasized that the functional morphology of most of the setae can be better understood when the whole organ is studied of which the setae are only a component. Not all the setae have an adaptive significance. A descriptive model of the chaetotaxy of cypridacean ostracods is presented. The different characteristics of the setae as well as their position on the limbs are coded by letters and numerals using simple formulae.