Public drinking water that is derived from surface water requires treatm ent to remove unwanted micropollutants, pathogenic microorganisms and growth-supporting nutrients. The intended result is water that is suitable for human consumption and that can be stored and/or distributed without problems such as biofouling, adverse taste and odor development, or pathogen growth during distribution. Biofiltration is an important process during the treatment of drinking water, either as part of a centralized multistep treatment system in industrialized countries, or as a decentralized single-step treatment process in remote areas (Figure 1 ). Biofiltration is defined as a filtration process where the filter medium comprises porous material (e.g., sand, granulated active carbon (GAC), or a synthetic carrier material), which is colonized by indigenous microbial communities and where the micro-organisms perform at least one of the essential treatment functions of the process. The first purpose of biofiltratio n in drinking water treatment is to achieve removal of biodegradable organic matter (BOM) and the second purpose is the elimination of pathogenic microorganisms, while additional functions such as ammonia, nitrite, and manganese removal have previously been ascribed to a variety of biofiltration systems [ 1– 3] . The latter systems are addressed in article 495 “attached growth in biological systems in the treatment of potable water and wastewater” by D.V. Vayenas (this volume). The broader concept of drinking water biofiltration includes (semi)natural systems such as riverbank filtration (RBF) and soil aquif er treatment (SAT), as well as mechanical/engi neered systems such as rapid sand filters (RSFs), GAC filters, and slow sand filters (SSFs) [2, 4, 5]. This article covers only the mechanical/ engineered systems used predominantly in centralized drinking water treatment. Variants of the biofiltration process represent some of the oldest drinking water treatm ent practi ces in the world. Recorded references to the use of RBF date back to several millennia: “ and all the Egyptians dug along the Nile to get drinking water, because they could not drink the water of the river ” (The Bible, Exodus 7:24). The first industrial sand filtration systems in European drinking water treatment processes were pioneered about two centuries ago by John Gibb (Scotland, 1804) and James Simpson (England, 1829) [ 6]. However, the micro biological nature of biofilters was initially neither obvious nor necessarily intended. These first systems were installed as straining filters for turbidity removal and later used effectively for the containment of cholera, well before any definitive knowledge on bacteria in drinking water existed. It is common knowledge today that microorganisms are indigenous to drinking water treatment and distribution systems, where they can colonize nearly any surfac e and establish complex and vibrant communities in the low nutrient environment [1]. More importantly, within the context of drinking water treatment and biofiltration, bacterial presence is not only opportunistic or negative, but in fact forms the basis of treatment for the production of high-quality drinking water. This functional role of microorganisms in the treatment processes emerged about 50 years ago, with several studies investigating the presence and possible role of benign microbial communities in drinking water filters [6 ]. Understanding the microbial component of biofiltration is still an ongoing process, aided by the emergence of new and powerful tools for the analysis of microorganisms, an increased understanding of microbial growth in low nutrient environments, and the acknowledgment of drinking water as a unique, complex ecosystems. This article covers the state of the science of biofiltration, specifically focusing on mechanical/engi neered systems, and particularly on the role of microorganisms in the treatment process.