Trickling filters are a fundamental technology in biological wastewater treatment, valued for their straightforward operation, reliability, and efficiency in reducing organic pollutants. This chapter examines the performance of trickling filters, with a focus on their effectiveness in removing contaminants such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), and total dissolved solids (TDS) from both domestic and industrial wastewater. The treatment process centers on a biofilm of microorganisms that develops on the filter media. As wastewater flows over this surface, the biofilm breaks down organic matter, resulting in significant reductions in pollutant levels. Trickling filters typically achieve high removal rates for BOD and COD, especially at lower flow rates where there is more contact time between the wastewater and the biofilm. The removal of suspended solids is also notable, while reductions in dissolved solids are more moderate and depend on operational conditions. Factors such as flow rate, temperature, and the accumulation of biomass can influence performance, but trickling filters generally maintain stable operation under varying conditions. The chapter also discusses the adaptability of trickling filters to different loading rates and their ability to handle fluctuations in wastewater quality. These systems require relatively low energy and maintenance compared to other biological treatment methods, making them suitable for a wide range of applications. Innovations in filter media, including the use of sustainable materials, have further enhanced the durability and effectiveness of trickling filters. Trickling filters provide a robust, cost-effective, and environmentally friendly solution for secondary wastewater treatment. Their performance is shaped by the interaction of wastewater characteristics, biofilm activity, and system design, ensuring their continued importance in modern wastewater management.