Biofilms are attached forms of bacteria and other microorganisms enclosed in a matrix of extracellular polymeric substances (EPS), and comprise a microbial lifestyle that is quite different from that of free-living planktonic cells. The biofilm state is now universally-recognized for its complexity and resiliency to stresses, and importance in natural environments, as well its roles in comensal flora and infection processes. However, the EPS matrix, which occur just 'outside of cells', is poorly understood, and has been understated in the literature. Yet this extracellular milieu is crucial to the functioning and resiliency of the biofilm. Recently, exciting new advances have emerged that are helping to understand the EPS matrix, its processes, ultrastructure, and importance to cells in nature and disease.