<p>Congenital heart defects encompass a wide spectrum of varying anatomies and physiologies. Residuals after surgical or interventional treatment during childhood significantly shape the clinical course, as do multiple organ involvement and the development of degenerative diseases. As adults, many patients with congenital heart defects are no longer treated within specialized care structures, which is associated with a worsening prognosis and an increased frequency of emergency situations. Without appropriate expertise, emergencies in patients with complex congenital heart defects are challenging to assess. Arrhythmias and heart failure are the most common emergency scenarios in adults with congenital heart defects. Atrial tachycardias, particularly in the context of a Fontan circulation or a systemic right ventricle, should be treated promptly. Since the overall risk of sudden cardiac death is increased, syncope must always be thoroughly investigated. Depending on the specific anatomy and physiology, different therapeutic strategies must be employed for heart failure. In Eisenmenger syndrome, chronic cyanosis with reactive polycythemia and multiple organ involvement is the primary concern. Pregnancy and the peripartum period may precipitate decompensation of previously compensated conditions in patients with congenital heart disease. The ABCDE approach (Airway, Breathing, Circulation, Disability, Exposure) can generally be followed. However, without knowledge of the patient’s medical history, treatment errors with potentially severe consequences can easily occur. By adding the letter F to the ABCDE scheme, the importance of the patient’s Former medical history and additional information (Further information) in this vulnerable patient group is emphasized. A multidisciplinary approach and close coordination with a specialized adult congenital heart disease (ACHD) center are critical for improving the prognosis in acute situations.</p>