Many filamentous nitrogen-fixing cyanobacteria protect nitrogenase from oxygen in differentiated cells called heterocysts. Heterocyst development is controlled by the availability of nitrogen compounds in the environment and by intrinsic factors that regulate the frequency and pattern of heterocysts along vegetative cell filaments. Recent progress in understanding heterocyst development in these simple multicellular organisms includes demonstrating the role of 2-oxoglutarate in regulating the activity of the transcription factor NtcA, the identification of additional genes in the regulatory network, such as hetF, and the further characterization of previously identified genes and proteins, including DevR/HepK, hetR, hetN, patS and patB.