There is agreement among ecology researchers that the concepts of legitimation and competition are partly responsible for organizational selection. Finding adequate measures to represent these concepts, however, has been elusive and has been the main obstacle to the development of this stream of investigation. On the basis of the germinal density dependence model, we identify two lines of research, which have generated seven distinct models. This survey shows that there has been a general tendency to use only three variables to measure legitimation and competition, which are often measured together. However, we argue that fruitful results have emerged when efforts have been made to separate both concepts and that there is some potential in using new measures (mass or concentration) which have so far only found limited application in the field of population demography. These findings, together with the inconclusive results of this stream of research, allow us to identify the existing gaps in the literature and comment on directions for future research.