Electrochemical energy technologies will significantly contribute to the transition to a sustainable and carbon-neutral economy in order to decarbonize the energy sector. One of the most critical scientific and technological problems is the development of a sustainable and cost-effective water splitting processes for industrial-scale production of green H2 to facilitate the energy transition. Heterogeneous catalysis is a key element of the solution. Benchmark electrocatalysts with a zero HER overpotential used in low temperature water electrolysis systems are, however, based on scarce, expensive noble metals (specifically platinum group metal (PGM) catalysts). This makes them economically less attractive and does not meet the requirements for the global demand of green H2. Consequently, the development of low-cost, stable, and highly efficient hydrogen-evolving materials is of key importance to meet this demand. Earth-abundant transition metal (TM)-based materials have attracted much attention as alternative electrocatalysts, especially in alkaline electrolytes. However, chemical modulation strategies of catalysts and economically viable synthesis routes to generate nanostructured materials with durable and enhanced catalytic performances are still required especially for the green hydrogen production, especially at an industrial scale. A major process bottleneck, when operating in alkaline media, is the reduced kinetics due to the water-dissociation energy barrier. However, alkaline water electrolysis allows the utilization of TM-based materials, which serve as PGM-free alternatives with better HER kinetics. Emerging technologies such as anion-exchange membrane water electrolysis, for industrial scale water splitting, require electrocatalysts that are capable to even operate under mild alkaline conditions (less or equal to 1M KOH) or pure water. To enhance the kinetics of the reduction reaction, the chemical environment of TM-based catalysts can be tailored by incorporating nonmetals and metalloid elements such as phosphorus, sulfur, nitrogen, selenium, and boron. Engineering of the material's morphology, crystal structure, and catalyst support are leveraged to improve extrinsic catalytic activity of complete electrodes. A careful choice of the substrate material and the design of the substrate/catalyst interface especially for highly conductive substrates is also important. This enables strong adhesion and enhanced charge transfer due to extended exposure of more catalytic active sites to adsorbates promoting the HER activity and life-cycle of the electrocatalysts. By combining the above strategies, earth-abundant transition metal-based catalysts are developed with high activity comparable to precious metal-based catalysts.