Hydrogen, as an abundant and clean, renewable energy source, has taken the center stage to replace carbon-based fuels. Among hydrogen generation strategies, electrochemical water splitting through hydrogen evolution reaction (HER) is a green and efficient approach for hydrogen production. However, industrializing this technology is not yet efficient enough, due to the high expenses of needed metals such as platinum. Recently, transition metal borides (TMBs) have been proposed as potential electrocatalysts for HER applications, both for bulk and nanoscale materials, owing to their low cost, high activity, and stability in acidic and basic electrolytes. Meanwhile, some studies indicate that boron covalent networks in crystal structures play an important role in electrocatalytic activity. Therefore, further research on structure-activity relationships is required to understand and possibly even predict more active and efficient catalysts. Herein, we propose the possible application of transition metal diborides as electrochemical catalysts for HER. Firstly, the high activity of VxBy has been rationalized by studying the effect of aggregating boron chains as a function of HER activity. A structure-activity relationship is found that helps to predict new phases and HER overpotentials. Then, we report on an unusual volcano-like behavior of the c lattice parameter in the AlB2-type solid solution V1-xMoxB2 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) and its direct correlation to the HER activity in 0.5 M H2SO4 solution. Further, a more detailed structure-activity relationship in transition metal boride studied in the lab since 2017 is summarized. In addition to the transition metal boride, we apply Mo2BC to HER electrocatalyst in both acid and alkaline electrolytes for the first time and suggest the synergy effect of β-MoB and α-MoC substructure in Mo2BC crystal structure. Moreover, a suitable electrode preparation method beyond dense electrodes, which was studied in our previous report, is proposed for maximizing HER activity. Finally, we report on the experimental and theoretical investigations of the HER activity of Fe5-xGe2Te2. It is a recently discovered iron-richer phase that is structurally related to Fe3GeT2.