Advances in the fabrication and characterization of nanowires polytypes have made crystal phase engineering a well-established tool to tailor material properties. In this review, recent progresses in the field are described, with special focus on the central role that crystal phase has in modulating the electronic and optical properties of nanowires. We start with an overview on III-V nanowires, which have been developed first and for which more experimental results already exist. Then, particular attention will be devoted to group IV polytypes which represent one of the most exciting and novel topic in the field. A survey of both theoretical and experimental efforts will be provided with the final aim of highlighting the importance of crystal phase control in materials design.