A series of 15 stages of development for the mouse limb bud have been defined, spanning the time from the first appearance of the limb bud to the completion of limb outgrowth. The stages are based on changes in the morphology of the limb in living preparations. The development and regression of the apical ectodermal ridge (AER) as well as the development of the skeletal structures are also described. This staging system has been developed in response to the need to standardize in situ experimental analyses of the mouse limb bud. Comparable stages of the commonly used chick wing and mouse whole embryo systems are presented.