Hydrogen sulfide (H
2
S) has emerged as a gaseous signalling molecule with crucial implications for cardiovascular health. H
2
S is involved in many biological functions, including interactions with nitric oxide, activation of molecular signalling cascades, post-translational modifications and redox regulation. Various preclinical and clinical studies have shown that H
2
S and its synthesizing enzymes — cystathionine γ-lyase, cystathionine β-synthase and 3-mercaptosulfotransferase — can protect against cardiovascular pathologies, including arrhythmias, atherosclerosis, heart failure, myocardial infarction and ischaemia–reperfusion injury. The bioavailability of H
2
S and its metabolites, such as hydropersulfides and polysulfides, is substantially reduced in cardiovascular disease and has been associated with single-nucleotide polymorphisms in H
2
S synthesis enzymes. In this Review, we highlight the role of H
2
S, its synthesizing enzymes and metabolites, their roles in the cardiovascular system, and their involvement in cardiovascular disease and associated pathologies. We also discuss the latest clinical findings from the field and outline areas for future study.