This chapter explains the fundamental scientific concepts underpinning two major purification mechanisms: adsorption and photocatalysis. Adsorption principles, kinetics, isotherms, and thermodynamic parameters are discussed alongside common sorbents, including natural materials, industrial wastes, biomaterials, and engineered nanomaterials. Photocatalysis is explored through its charge-transfer mechanisms, reactive oxygen species formation, and performance-enhancing strategies. This chapter also introduces integrated photocatalyst–adsorbent (IPCA) systems, describing selection criteria and synergistic effects relevant to practical wastewater treatment. The content builds a strong theoretical basis for the material-specific chapters that follow.