The trickling filter solids contact (TF/SC) process is now over 20 years old, having been first demonstrated successfully in 1979 at Corvallis, Oreg. Design and operating concepts have evolved significantly since the first generation of plants. The purpose of this paper is to document the changes that have occurred since prior reviews. Data from four large plants (Santa Cruz, Littleton/Englewood, Central Valley, and Annacis Island) are used for illustration. The basis of the original peer reviewed U.S. EPA definition of the TF/SC process is reviewed. Pilot-scale and plant-scale research coupled with advanced analysis tools had allowed the overall horizons of the TF/SC process to be expanded, such that higher loadings have been obtained. For instance, use of modern plastic media has allowed the criterion that the majority of the biochemical oxygen demand be removed on the trickling filter while sustaining high total organic loadings. Owing to the features of the flocculator-clarifier and the superior thickening and compaction qualities of TF/SC solids, high solids loading rates can be obtained. As a result, current advanced designs occupy less footprint, and it is axiomatic that costs have been reduced. Furthermore, there is considerable potential to rerate, optimize, and debottleneck existing TF/SC plants to release low cost capacity.