With respect to multicellular evolution, cells occupy the gray area between genotype and phenotype. As a result, this level of biological organization has remained understudied. However, as we move beyond the "gene" to characterize the emergent properties that contribute to adaptive phenotypic variation, cells should figure more prominently. This chapter examines the roles of cells in morphological evolution. More accurately, it re-examines the recent evo-devo literature, with an eye not for genes and signal transduction pathways, but rather for the cellular behaviors these signals mediate. Most examples, taken from key events in animal evolution, illustrate the potential for cells and cell biology to expand the current evolutionary paradigm. The overall conclusion of this chapter is consistent with the general theme of this book, which is that cells mediate the genotype–phenotype connection, and therefore deserve to be brought more explicitly into the fold of modern evolutionary research and theory.