The use of Carbon Monoxide (CO) as a therapeutic agent has already been tested in human clinical trials. Pre-clinically, CO gas administration proved beneficial in animal models of various human diseases. However, the use of gaseous CO faces serious obstacles not the least being its well-known toxicity. To fully realise the promise of CO as a therapeutic agent, it is key to find novel avenues for CO delivery to diseased tissues in need of treatment, without concomitant formation of elevated, toxic blood levels of carboxyhemoglobin (COHb). CO-releasing molecules (CO-RMs) have the potential to constitute safe treatments if CO release in vivo can be controlled in a spatial and temporal manner. It has already been demonstrated in animals that CO-RMs can release CO and mimic the therapeutic effects of gaseous CO. While demonstrating the principle of treatment with CO-RMs, these first generation compounds are not suitable for human use. This tutorial review summarises the biological and chemical behaviour of CO, the current status of CO-RM development, and derives principles for the creation of the next generation of CO-RMs for clinical applications in humans.