Carbon monoxide (CO) is a colourless and odourless gas that has long been considered as a potent respiratory poison. Recent advances have demonstrated its production by haem oxygenases in both mammals and microbes, and it has roles as a gasotransmitter in higher organisms. This review concentrates on the application of CO, via carbon monoxide-releasing molecules (CO-RMs), as an anti-bacterial agent. Currently, the scope of literature on the effects of CO on bacteria is small, and we have included discussions on the production of CO by bacteria via haem oxygenase enzymes, the use of CO as an energy source, and existing knowledge on CO sensors in bacteria. CO is known to target haem proteins and is an effective inhibitor of respiration, even when provided at concentrations much higher than prevailing oxygen. We review here data suggesting that CO-RMs are more effective inhibitors of respiration than is CO gas, perhaps due to the ability of CO-RMs to deliver CO selectively to intracellular targets. We also consider the recently reported transcriptomic consequences of CO-RM treatment of Escherichia coli, revealing a myriad of unexpected targets for CO and potential CO sensors. Finally, we consider the use of CO and CO-RMs as anti-bacterial agents in vivo, and the future prospects for this gaseous molecule.