A model is a representation of or an analogy for something else; in a biological context, the term often refers to organisms or species that serve as a widely used platform for experimental research. In developmental biology, an extraordinarily detailed understanding of fundamental genes and mechanisms has been built around a few core models including Drosophila, mouse, Caenorhabditis elegans, Arabidopsis, and zebrafish. The use of model species entails a series of epistemological issues and commitments regarding similarity, generalization, and the balance between representation and accessibility. In addition, the power of a given model is highly context-dependent, and the core models of developmental biology may not suffice for evo-devo questions. An epistemological perspective can clarify the complementary roles of standard and emerging models in evo-devo, as well as inform decisions about when new models are needed, and criteria for choosing them.