Azacitidine (Vidaza(®)) is a pyrimidine nucleoside analogue of cytidine and is approved in the EU for use in patients with higher-risk myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), including older patients (aged ≥65 years) with AML with >30 % bone marrow blasts (BMB) who are ineligible for haematopoietic stem cell transplant. This article reviews the clinical efficacy and tolerability of azacitidine in the treatment of these patient populations, as well as summarizing its pharmacological properties. In pivotal, international, phase 3 trials, subcutaneous azacitidine was an effective and well tolerated treatment in patients with higher-risk MDS or AML, including older patients with AML with >30 % BMB, with extensive evidence from the real-world setting confirming its efficacy and safety in these patient populations. Azacitidine is the only approved hypomethylating agent that has been shown to prolong overall survival compared with conventional care regimens and thus, it is recommended as the first-line hypomethylating agent for most patients with higher-risk MDS. Hence, azacitidine remains and important agent for use in the treatment of higher-risk MDS and AML, including in older patients with AML with >30 % BMB.