Book excerpt: Aiming at the generation of hydrogen from water, electrochemical water splitting represents a promising clean technology for generating a renewable energy resource. The book reviews the fundamental aspects and describes recent research advances. Properties and characterization methods for various types of electrocatalysts are discussed, including noble metals, earth-abundant metals, metal-organic frameworks, carbon nanomaterials and polymers. Keywords: Electrochemical Water Splitting, Renewable Energy Resource, Electrocatalysts, Oxygen Evolution Reaction (OER), Noble Metal Catalysts, Earth-Abundant Metal Catalysts, MOF Catalysts, Carbon-based Nanocatalysts, Polymer Catalysts, Transition Metal-based Electrocatalysts, Fe-based Electrocatalysts, Co-based Electrocatalysts, Ni-based Electrocatalysts, Metal Free Catalysts, Transition-Metal Chalcogenides, Prussian Blue Analogues.