Stellar Imager is a mission concept for a space-based ultraviolet/visible light Interferometer to image and study the surfaces of stars, with more than 200 times the resolution of the Hubble Space Telescope. The mission's primary objective is to provide data needed to develop and validate a predictive dynamo model for the sun and other magnetically active stars. The instrument would achieve this objective by characterizing the patterns of surface magnetic activity for a large sample of sun-like stars; characterizing the internal structure and differential rotation of these stars; and determining the dependence of dynamo action on mass, internal structure, flow patterns, and time by carrying out a population study of sun-like stars.