SYNOPSIS. The embryonic chick limb bud is a growing organ rudiment whose undifferentiated cells give rise to a precise spatial pattern of differentiated structures. The establishment of positional values of chick limb bud cells (pattern specification) and the response of limb bud cells with established positional values to experimental perturbations (pattern regulation) are the major topics considered in this paper. The results of recent experiments with developing chick limb buds analyzing pattern specification and pattern regulation are presented. These studies with the chick limb are described in light of the postulates of a model that was originally formulated from experiments performed on regenerating amphibian and insect appendages.