A biofilm is a community of microorganisms attached to a surface. Based on studies of single-species communities, biofilm formation follows a progression from initial attachment to a mature form composed of pillar-like multicellular structures interspersed with fluid-filled channels. The developmental progression leading to a mature biofilm requires changes in gene expression. With recent technological advances for visualizing biofilm growth, gene expression can be directly monitored during biofilm development. Hence, analyses of surface-induced and biofilm-induced changes in gene expression have begun in earnest. Recent studies have identified regulatory pathways that are important for biofilm formation and have focused on genetic responses to environmental stimuli in mature biofilms. These findings are providing new insights into biofilm development and physiology.