Insulin and the insulin-like growth factors (IGF-I, IGF-II) are pleiotropic hormones that have multiple roles in regulating vital metabolic and developmental processes. Although most early data suggested that insulin is mainly involved in metabolic activities (e.g. control of sugar levels) and IGF-I/II control growth and differentiation events (e.g. bone elongation, cell division), today, it is clear that there is cross-talk between the various ligands and receptors of the IGF family. As a result of these complex interactions, the spectrum of activities that were classically assigned to insulin or IGF-I/II has greatly expanded, and the signalling events mediated by the insulin and IGF receptors is the subject of intensive research. This review provides a comparative analysis of the structures, receptors, and signalling pathways of insulin and IGF-I.