Acute heart failure (AHF) is a complex syndrome characterized by worsening heart failure (HF) symptoms that requires escalation of therapy. Intrinsic cardiac abnormalities and comorbid conditions, including lung and renal disease, and sleep-disordered breathing, can contribute to the development of AHF. In this Review, we summarize and discuss the literature on the clinical evaluation and underlying pathophysiology of AHF. Important features of AHF evaluation include identification of precipitating factors to the disease, and assessment of circulatory-renal limitations associated with use of HF medications, prior HF hospitalizations, congestion and perfusion profiles, and end-organ dysfunction. The pathophysiological contributions of endothelial dysfunction, neurohormonal activation, venous congestion, and myocardial injury to the development of AHF are also discussed. These potential causative mechanisms provide a framework for clinicians to evaluate and manage patients with AHF and highlight possible future targets for therapies designed to improve clinical outcomes.