Due to the inherent chemical disorder and the potential for tunable degrees of short-range order in these materials (SRO), the process of deformation substructure evolution in general, and slip-to-twin transition in particular, in medium entropy alloys (MEAs) is expected to deviate from that in conventional alloys. For FCC materials, after having previously developed a methodology for understanding deformation substructure formation in pure Cu, the group has turned its attention to the CoCrNi MEA system. In this talk, we relate processing parameters to the rise of SRO and ultimately to dynamic substructure development and mechanical behavior. Microstructures of HEAs subjected to a variety of deformation regimes and quantified using diffraction-based techniques. The extent to which SRO controls localized dislocation-based phenomena during microstructural evolution is discussed. The techniques presented allow for the direct observation of the interplay between chemistry and microstructure, and thus, provides us with key tuning knobs for future HEA development.