Stellarators are non-axisymmetric magnetic field configurations for confinement of fusion plasmas. In contrast to the more popular axisymmetric tokamak geometries, stellarators rely on symmetry breaking to confine particles. The Simons Collaboration on Hidden Symmetries and Fusion Energy is a team dedicated to advancing the mathematical and computational state of the art in stellarator design, and producing modern stellarator optimization codes and new underlying theory. In this talk, I describe some of the challenges of stellarator optimization, and give examples both of success stories and of challenges that remain.