Giant strides have been made in identifying the molecular basis of limb development. The four main phases are initiation of the limb bud, specification of limb pattern, differentiation of tissues and shaping of the limb, and growth of the miniature limb to the adult size. We will focus on the exciting advances that have been made in initiation and specification of limb pattern. The limb is a model system and the same sets of molecules are used at different times and places in vertebrate embryos. There is also remarkable conservation of the molecular mechanisms of limb development in insects and vertebrates.