ABSTRACT During the past decade we have gained much knowledge about the molecular mechanisms that are involved in initiation and termination of biofilm formation. In many bacteria, these processes appear to occur in response to specific environmental cues and result in, respectively, induction or termination of biofilm matrix production via the second messenger molecule c-di-GMP. In between initiation and termination of biofilm formation we have defined specific biofilm stages, but the currently available evidence suggests that these transitions are mainly governed by adaptive responses, and not by specific genetic programs. It appears that biofilm formation can occur through multiple pathways and that the spatial structure of the biofilms is species dependent as well as dependent on environmental conditions. Bacterial subpopulations, e.g., motile and nonmotile subpopulations, can develop and interact during biofilm formation, and these interactions can affect the structure of the biofilm. The available evidence suggests that biofilm formation is programmed in the sense that regulated synthesis of extracellular matrix components is involved. Furthermore, our current knowledge suggests that biofilm formation mainly is governed by adaptive responses of individual bacteria, although group-level activities are also involved.