Glutamine is a metabolite of central importance in bacterial physiology. In addition to its function as one of the 20 standard amino acids in protein synthesis, glutamine is required for the biosynthesis of a variety of nitrogen-containing compounds. Of particular importance is glutamine synthesis as primary reaction of ammonium assimilation. Because of this versatile role, glutamine metabolism is tightly controlled in response to the cellular nitrogen status in bacteria. Recent progress in elucidating the molecular basis of nitrogen signalling has shed light on the role of glutamine as a signalling molecule. Bacteria belonging to the phylogenetic domains of proteobacteria and low G+C gram-positives (firmicutes) have evolved different mechanisms to monitor glutamine as an indicator of the state of nitrogen metabolism, which then regulates nitrogen metabolism at the transcriptional and post-transcriptional levels. Using the conserved P_II signal transduction system, major groups of prokaryotes, including the cyanobacteria, have evolved yet another strategy to monitor the cellular nitrogen status, which relies on 2-oxoglutarate instead of glutamine as the signalling molecule. In addition to monitoring the intracellular glutamine level, bacteria may respond to extracellular glutamine, which is used as a nutrient. This overview details our current knowledge of glutamine-regulated processes in bacteria.