How limb size and shape is regulated is a long-standing question in developmental and regeneration biology. Recently, the protocadherin Dachsous-Fat (Ds-Ft) signaling pathway has been found to be essential for wing development of the fly and leg regeneration of the cricket. The Ds-Ft signaling pathway is linked to the Warts-Hippo (Wts-Hpo) signaling pathway, leading to cell proliferation. Several lines of evidence have suggested that the Wts-Hpo signaling pathway is involved in the control of organ size, and that this pathway is regulated by Ds-Ft and Merlin-Expanded, which are linked to morphogens such as decapentaplegic/bone morphogenic protein, Wingless/Wnt, and epidermal growth factor. Here we review recent progress in elucidating mechanisms controlling leg size and shape in insects and vertebrates, focusing on the Ds-Ft signaling pathway. We also introduce a working model, Ds-Ft steepness model, to explain how steepness of the Ds-Ft gradient controls leg size along the proximodistal axis. Developmental Dynamics 240:1028-1041, 2011. (C) 2011 Wiley-Liss, Inc.