Phenotypic accommodation is adaptive adjustment, without genetic change, of variable aspects of the phenotype following a novel input during development. Phenotypic accommodation can facilitate the evolution of novel morphology by alleviating the negative effects of change, and by giving a head start to adaptive evolution in a new direction. Whether induced by a mutation or a novel environmental factor, innovative morphological form comes from ancestral developmental responses, not from the novel inducing factor itself. Phenotypic accommodation is the result of adaptive developmental responses, so the novel morphologies that result are not “random” variants, but to some degree reflect past functionality. Phenotypic accommodation is the first step in a process of Darwinian adaptive evolution, or evolution by natural selection, where fitness differences among genetically variable developmental variants cause phenotype-frequency change due to gene-frequency change. J. Exp. Zool. (Mol. Dev. Evol.) 304B, 2005. © 2005 Wiley-Liss, Inc.