The filamentous, heterocyst-forming cyanobacteria are among the simplest multicellular prokaryotes, and Anabaena sp. strain PCC 7120 is being used as a model for studying multicellularity in these organisms. In the absence of combined nitrogen two interdependent cell types are present in an Anabaena filament: vegetative cells and heterocysts. Vegetative cells perform oxygenic photosynthesis and supply carbon compounds to the heterocysts, which are specialized in the assimilation of atmospheric N(2) and supply nitrogenous compounds to the vegetative cells. In this chapter, we discuss two possible pathways for the exchange of metabolites and regulatory signals between vegetative cells and heterocysts: the continuous periplasm that surrounds the cells in the filament and some septal proteinaceous complexes that could allow the direct intercellular transfer of small molecules.