Numerous causes contribute to water pollution, including homes, industry, mining, and infiltration; however, one of the most significant is the extensive utilization of water by industrial activities. Four types of water are often identified: (1) Rainfall (runoff from impermeable surfaces), (2) household waste, (3) Irrigation water, and (4) wastewater from factories. The last category can be separated into cooling fluid, washing effluent (of varied composition), and production or process water (biodegradable and/or possibly hazardous). Generally, processing waters presents the most significant challenges. Wastewater markedly differs from potable water sources (often rivers, lakes, or reservoirs) in a crucial aspect: contaminant levels in most of the water for consumption sources are far lower than those in effluents originating from industrial activity. Nonetheless, their harmful effects are dependent upon composition, which is influenced by their industrial source. This chapter provides the in-depth discussion of conventional and advanced technqiues for dye degradation. Conventional methods include physical (nanofiltration, adsorption, coagulation), chemical (advanced oxidation processes such as UV, ozone, and Fenton-based treatments), and biological (aerobic and anaerobic processes using biofilms). Advanced techniques including catalytic and photocatalytic processes were thoroughly discussed. Lastly, the mechanism of photocatalysis with influencing factors (irradiation time, effect of oxygen hydroxide, catalyst dosage, and, pH) were described.