The most important function of Leydig cells in the testis is the synthesis and secretion of androgens, essential for the development of sex organs, secondary sex characteristics and normal progression of spermatogenesis. In general, two phases of Leydig cell development are identified, a foetal phase and a postnatal phase, distinguished by morphological and functional characteristics. In this chapter, we provide an overview of the morphological differences in foetal and postnatal Leydig cell (ultra)structure and relate this to the function (androgen synthesis) of the cells in specific phases of development. Although in the past two decades most studies have focused on Leydig cell development and function in rodents, there is a vast amount of older literature available that describes this process in other species. These studies contain careful descriptions of histological and ultrastructural changes in Leydig cells developing along the Leydig cell lineage. An analysis is provided of similarities and differences in Leydig cell morphology and function throughout development in rodents, primates, pigs, sheep and rabbits. Hypotheses related to the origin of the foetal and postnatal Leydig cell populations based on morphological observations and functional aspects are aligned to modern lineage tracing studies and transcriptomic analysis. Interestingly, some of the older hypothesis related to the origin of the foetal and postnatal Leydig cell populations are confirmed and extended by these tracing studies. Examples of these are the involution of foetal Leydig cells in the second half of pregnancy, followed by redifferentiation of part of these cells into adult Leydig cells round puberty, and the persistence of foetal Leydig cells in the adult testis. Taken together, the combination of classical morphological and functional studies with modern tracing and transcriptomic analyses support each other and contribute to our understanding of the origin and development of Leydig cells along the Leydig cell lineage.