The limb bud is of paradigmatic value to understanding vertebrate organogenesis. Recent genetic analysis in mice has revealed the existence of a largely self-regulatory limb bud signalling system that involves many of the pathways that are known to regulate morphogenesis. These findings contrast with the prevailing view that the main limb bud axes develop largely independently of one another. In this Review, we discuss models of limb development and attempt to integrate the current knowledge of the signalling interactions that govern limb skeletal development into a systems model. The resulting integrative model provides insights into how the specification and proliferative expansion of the anteroposterior and proximodistal limb bud axes are coordinately controlled in time and space.