In this lecture an attempt has been made to apply our present knowlegde in comparative ethology to unravel the processes leading to sexual identity and partner selection. An emphasis is laid on the phenomenon of genetically programmed learning. A survey is given of the kind of knowlegde gathered by ethologists in this field, mostly on birds and mammals. On the basis of this knowlegde and of data published by psychiatrists a very tentative and speculative hypothesis is constructed of how the ontogenetical development of sensory and motoric aspects of sexual behaviour might take place and where and how failures in the performance of the (species-specific) programme might lead to deviations in the development. Once a deviation has taken place it is likely to lead to another when a following critical phase has to be passed. The main purpose of this survey is to stimulate the study of the development of sexual behaviour in the human species, in addition to other methods, also by a typical ethological approach.