PDE type 5 inhibitors (PDE5Is), such as sildenafil, tadalafil and vardenafil, are a class of drugs used to prolong the physiological effects of NO/cGMP signalling in tissues through the inhibition of cGMP degradation. Although these agents were originally developed for the treatment of hypertension and angina, unanticipated side effects led to advances in the treatment of erectile dysfunction and, later, pulmonary arterial hypertension. In the last decade, accumulating evidence suggests that PDE5Is may confer a wider range of clinical benefits than was previously recognised. This has led to a broader interest in the cardiovascular therapeutic potential of PDE5Is, in conditions such as hypertension, myocardial infarction, stroke, peripheral arterial disease, chronic kidney disease and diabetes mellitus. Here, we review the pharmacological properties and established licensed uses of this class of drug, along with emerging therapeutic developments and possible future indications. 1 | INTRODUCTION Few other pharmaceutical agents have had a history as serendipitous as the class of PDE type 5 inhibitors (PDE5Is; Figure 1). Developed over 30 years ago, initially for cardiovascular indications, PDE5Is emerged as a revolutionary treatment for erectile dysfunction (ED), licensed in 1998 (Goldstein, Lue et al., 1998). Further investigation of their effects on pulmonary arterial pressure culminated in their approval for the treatment of pulmonary arterial hypertension (PAH) in 2005 (Galiè, Ghofrani et al., 2005). Early reports of fatal cardiovascular events in patients prescribed a PDE5I delayed wider research into their therapeutic benefits, but it soon became apparent that these adverse events reflected the complex nature of patients initially receiving the drug and their high level of cardiovascular risk, rather than the properties of the drug itself (Cheitlin, Hutter et al., 1999;