Power systems are transitioning from synchronous generator (SG) dominated operation to fleets of inverter-based resources (IBRs), with grid-forming (GFM) control as the leading candidate for preserving voltage-source behavior. Yet GFM inverters deviate from the ideal voltage source in ways that undermine stability, spanning non-passive high-frequency behavior, poorly damped line-frequency oscillations, loss of synchronism under current limiting, and switching-induced distortion. Moreover, inverters from different vendors lack a behavioral contract, hindering stable interoperation at scale. This thesis addresses both problems. At the device level, it develops and experimentally validates controllers that resolve each instability mechanism, including delay compensation, angle restoration control, and power-angle-aware current limiting. At the system level, it synthesizes these results into a rules-and-signaling framework: a minimal, explicit behavioral contract under which GFM IBRs from different vendors interoperate stably without centralized coordination, much as SGs converged on droop and inertia over decades. The result is a path toward certifiable, vendor-heterogeneous GFM fleets.