Uncontrolled plasma shutdowns in tokamaks have the potential to generate intense beams of relativistic (multi-MeV) “runaway” electrons, which if not safely dissipated pose a severe risk to the integrity of plasma-facing components. These beams are distributed in physical space as well as in phase space, presenting complex dependencies for both theory and experiment. A new diagnostic capable of resolving these multi-dimensional distributions has recently been deployed on the DIII-D tokamak. Called the “gamma-ray imager” (GRI), this diagnostic employs multi-view pulse-height counting of bremsstrahlung-emitted gamma rays from the relativistic electrons. This seminar will introduce the diagnostic, describe its first measurements of relativistic electron distributions and their dependence on plasma parameters, and finally discuss how these measurements will help validate theoretical models of relativistic electron control for extrapolation to burning plasmas.