Solar water splitting, through electrocatalysis, photoelectrocatalysis, and photocatalysis, presents a promising pathway for converting abundant solar energy into clean hydrogen fuel. However, to meet the increasing demand for green hydrogen, catalysts for the water splitting reaction must not only achieve high efficiency but also be primarily composed of abundant materials with minimal environmental impact. Understanding the interplay between surface properties, optical properties, and chemical reactivity is crucial to identify suitable alternative materials capable of achieving high solar-to-hydrogen conversion. In this chapter, we discuss the fundamentals of electrochemical, photoelectrochemical, and photocatalytic water splitting. We explore how material’s properties and surface characteristics influence the hydrogen evolution reaction and oxygen evolution reaction. We revise their working principles and key concepts, from light absorption and charge separation to surface characteristics and adsorption energies. Additionally, we discuss key physical, chemical, and optical properties of electrodes and photoelectrodes that are crucial for an efficient water splitting, while presenting strategies for achieving high performance.