Bacteria form multicellular biofilm communities on most surfaces. Genetic analysis of biofilm formation has led to the proposal that extracellular signals and quorum-sensing regulatory systems are essential for differentiated biofilms. Although such a model fits the concept of density-driven cell-cell communication and appear to describe biofilm development in several bacterial species and conditions, biofilm formation is multifactorial and complex. Hydrodynamics, nutrient load and intracellular carbon flux have major impacts, presumably by altering the expression of cellular traits essential for bacterial adaptation during the different stages of biofilm formation. Hence, differentiated biofilms may also be the net result of many independent interactions, rather than being determined by a particular global quorum sensing system.