Many bacteria are able to organize themselves into structurally complex communities known as biofilms. A key feature of biofilms is the presence of an extracellular polymeric gel-like matrix, synthesized by the bacteria, that gives the biofilm form and holds the constituent cells together. Despite much progress in understanding the genetic pathways that control the production of matrix, the specific signals that bacteria use to coordinate matrix synthesis remain poorly understood. In this talk I suggest that a general physical mechanism is used by the bacteria to coordinate the formation of biofilm structure as an alternative to specific biochemical mechanisms; in this proposed mechanism, the matrix itself is a cue that is sensed by the bacteria and used to coordinate matrix synthesis among the large population of cells forming the biofilm. I show that the increase in osmotic pressure generated by the polymeric matrix is sensed by the bacteria and used as a trigger to inhibit further synthesis of the matrix.