Heart failure is an increasing reason for hospitalization and the leading cause of death in patients with adult congenital heart disease (ACHD). Recently, the European Society of Cardiology and the American Heart Association published consensus documents on the management of chronic heart failure in ACHD patients. However, little data and/or guidelines are available for the management of (sub)acute heart failure. The ACHD population is heterogeneous by definition and often has complex underlying anatomy, which could pose a challenge to the physician confronted with the ACHD patient in (sub)acute heart failure. Recognizing the underlying anatomy and awareness of the possible complications related would result in better treatment, avoid unnecessary delays, and improve outcomes of the ACHD patient with (sub)acute heart failure. This review focuses on the management of (sub)acute heart failure in ACHD with specific attention to lesion-specific issues.