Hydrogen (H2) is one of the most important chemical commodities and is applied mainly for petroleum refining and synthesizing ammonia-based fertilizers and a critical part of a sustainable hydrogen economy. The electrochemical hydrogen evolution reaction (HER) coupled to renewable energy sources is a potential sustainable source of H2. Platinum is the most active HER catalyst, but high cost and scarcity limits its potential for widespread use. Our focus at SUNCAT in HER is two-fold: mechanistic understanding and accelerated catalyst discovery. Our fundamental efforts focus on the mechanistic analysis of hydrogen evolution various catalysts through the coupling of kinetic modelling ab initio electrochemical barriers. Our catalyst discovery efforts focus on the application of simple activity descriptors towards the development of earth-abundant, non-precious metal HER catalysts, such as metal alloys, transition metal phosphides and sulfides. Experimental efforts make use of a wide range of synthesis methods to control the atomic and nano-scale structure of the catalysts and in situ characterization methods under operating conditions.