Biofilm processes are used primarily to produce a biologically stable drinking water, which does not foster growth of microorganisms during its distribution. This article describes the characteristics of biofilms and biofilm processes. It emphasizes quantitative modeling of the phenomenon controlling the accumulation of biofilm and the removal of organic and inorganic materials comprising biological instability. The article describes a practical means, the normalized surface loading, for applying biofilm modeling to the design and analysis of biofilm processes. Special attention is given to the most common applications in drinking-water treatment: aerobic oxidation of low concentrations of biodegradable organic material, nitrification of ammonium nitrogen, and denitrification of nitrate nitrogen.