This chapter reflects on and makes explicit the distinctiveness of reasoning practices associated with model species in the context of evolutionary developmental research. Model species in evo-devo instantiate a unique synthesis of model systems strategies from developmental biology and comparative strategies from evolutionary biology. This synthesis negotiates a tension between developmental conservation and evolutionary modification to address scientific questions about the evolution of development and the developmental basis of evolutionary change. We review different categories of model systems that have been advanced to understand practices found in the life sciences in order to comprehend how evo-devo model species instantiate this synthesis in the context of three examples: the starlet sea anemone and the evolution of bilateral symmetry, leeches and the origins of segmentation in bilaterians, and the corn snake to understand major evolutionary change in axial and appendicular morphology.