A review with 21 refs. \"Biofilm\" describes microbial aggregates such as flocs, films, and sludges as preferred mode of life of many microorganisms. They all have in common that the organisms are embedded in a matrix of extracellular polymeric substances in which they can establish synergistic microconsortia. In this matrix they are protected against biocides. Furthermore the matrix acts as sorbent for nutrients, retains exoenzyme, and can be considered as a recycling yard for cellular components as well as for nutrients. Biofilms are ubiquitous and can be found even in extreme environments. In many cases they are dominated by bacteria and/or algae, but they can also harbor trophical levels and represent a nutrient source for protozoa and metazoa. Biofilms are system characterized by a vast spatial heterogeneity and temporal dynamics. Genes and signals are exchanged intensively. Thus, biofilms bear similarities to multicellular organisms.