The region of space at the periphery of cyanobacterial cells is the interface between the environment and intracellular processes. This metaspace may include a structure appressed to the outer wall and membrane, such as an extracellular polysaccharide (EPS), a structural and/or physiological discontinuity modulating metabolite fl ow, as well as a temporal fl ux that accompanies stress or cell division. The functional framework within this region is designed to recognize environmental perturbations and relay physical and biochemical information to the cell interior, and perhaps to the cell community, for the appropriate physiological response. Communication between the environment and the cells is thus initiated within this extracellular milieu, which is therefore an important spatial domain in cyanobacteria. The ECM of cyanobacterial cells is multifaceted. It is not only a complex and dynamic mixture of polysaccharides, proteins, cell remnants and lower molecular weight secondary metabolites, but a hyperspace that tunes seasonal as well as shortterm stochastic modulations in environmental conditions. Such stresses result in changes in both the composition and organization of the matrix as cyanobacterial cells adjust to the environmental perturbations. This chapter provides a critical appraisal of the ecology and evolution of the cyanobacterial ECM compared with other prokaryotes. Emphasis is placed on how little is understood about this “occupied space” and several hypotheses and examples are presented in an effort to promote additional investigations of this oft-ignored interface.