TITLE: What’s going on at optimal doping in high-temperature superconductors?” ABSTRACT: High magnetic fields are employed in many experiments to suppress high-temperature superconductivity and, thus, reveal the behavior of the underlying – and strikingly abnormal – normal state that gives rise to high temperature superconductivity in the cuprate and iron-based families of high-temperature superconductors. We will survey phenomena that the data from high magnetic fields suggest are linked to optimum doping, including a peak structure in the Hall number, quasiparticle mass enhancement*, and linear-magnetoresistance**.