Trickling filters (TF) have been a cornerstone of wastewater treatment for over 120 years. Through the dedicated efforts of multiple generations of professionals, trickling filters have evolved into a modern solution capable of meeting today’s wastewater treatment demands. Contemporary trickling filters can reach heights up to 12 m thanks to lightweight, high-performance plastic media. The introduction of advanced flow distribution and ventilation systems has significantly enhanced the performance of trickling filters compared to traditional rock filters. The scope of trickling filter technologies has expanded significantly from early BOD roughing to encompass complete carbon oxidation and nitrification. These filters can function as standalone biological treatment systems or be designed in stages to achieve various treatment levels. In the U.S., trickling filters are frequently integrated with activated sludge systems to leverage the benefits of both processes. This chapter covers the fundamentals of trickling filters and highlights major advancements over recent decades, including the development of plastic media, speed control distribution, and integrated trickling filter processes.