Abstract Acute heart failure (AHF) is a life-threatening emergency, which largely profits from early diagnosis and treatment. The prevalence of AHF is difficult to assess, estimates range between 1 and 12% in the general population. Despite recent therapeutic advances, in-hospital mortality is high with estimates varying from 4 and 18% in different registries. Due to large differences in AHF definitions and selection criteria AHF populations vary in their characteristics and outcomes. This is especially true for randomized clinical trials and the external validity of some of these trials is questionable. Additionally, the timing of data collection and/or initiation of new therapies vary with the setting of trials. The aim of this article is to call attention to the difference in AHF populations and to emphasize the need for research to clearly define these populations. AHF populations from registries and clinical trials are the basis for evidence-based management strategies. It is important that these populations represent the patients in whom these strategies will be applied in routine care.