Acute Myeloid Leukaemia (AML) is an aggressive and rare cancer of myeloid cells (a white blood cell responsible for fighting infections). Successful treatment of AML is dependent on what subtype of AML the participant has, and the age of the participant when diagnosed. Venetoclax is an experimental drug that kills cancer cells by blocking a protein (part of a cell) that allows cancer cells to stay alive. This study is designed to see if adding venetoclax to azacitidine works better than azacitidine on its own. This is a Phase 3, randomized, double-blind (treatment is unknown to participants and doctors), placebo controlled study in patients with AML who are >= 18 or more years old and have not been treated before. Participants who take part in this study should not be suitable for standard induction therapy (usual starting treatment). AbbVie is funding this study which will take place at approximately 180 hospitals globally and enroll approximately 400 participants. In this study, 2/3 of participants will receive venetoclax every day with azacitidine and the remaining 1/3 will receive placebo (dummy) tablets with azacitidine. Participants will continue to have study visits and receive treatment for as long as they are having a clinical benefit. The effect of the treatment on AML will be checked by taking blood, bone marrow, scans, measuring side effects and by completing health questionnaires. Blood and bone marrow tests will be completed to see why some people respond better than others. Additional blood tests will be completed for genetic factors and to see how long the drug remains in the body.