Titanium dioxide (TiO2) has emerged as one of the most widely studied photocatalytic materials due to its chemical stability, low cost, lack of toxicity, wide availability, biocompatibility, and strong oxidative activity under UV irradiation. However, its practical application in environmental remediation is limited by its wide band gap (~3.03–3.2 eV) and limited visible light absorption (275–405 nm). In recent years, doping and co-doping strategies have been extensively explored to improve the photocatalytic efficiency of TiO2 by enhancing its light absorption, charge separation, and surface reactivity. This chapter provides a comprehensive overview of the latest advancements in the doping and co-doping of TiO2 with metal and nonmetal ions. Special attention is given to the synthesis methods, structural and electronic modifications, and photocatalytic performance for the degradation of various organic and inorganic pollutants in water. Challenges, comparative analyses, and future perspectives on the rational design of doped TiO2-based photocatalysts are also discussed to guide further research and technological implementation.