Although the main goal of biological drinking-water treatment is production of a biologically stable drinking water, biological processes can also remove organic micropollutants that are of a health concern or that cause tastes and odors. Micropollutants are usually removed as secondary substrates, which means that their oxidation does not provide sufficient electrons or energy to support biomass growth and maintenance. This article develops the biochemical fundamentals and quantitative tools for describing the secondary utilization of micropollutants in biofilm processes. It connects the removals of the secondary substrates to the main goal of treatment, removal of biodegradable organic matter. The article critically reviews the biochemical potential for degrading micropollutants commonly found in drinking-water supplies: petroleum hydrocarbons, chlorinated hydrocarbons, and taste-and-odor compounds.