This book covers different technologies of water (photo)electrocatalysis for HER, OER or HPER, HOR and ORR, e.g., heterogenous and/or homogeneous photocatalysis, electrodes-based photoelectrocatalysis, electrocatalysis and photovoltaic-electrocatalysis. Mechanisms, materials, devices and systems of water (photo)electrocatalysis for energy conversion are intrinsically reviewed. This book also shows the principles are being used across these fields to develop technology with improved functionality and higher operating efficiency.