Advances in the management of congenital heart disease have resulted in a rapidly expanding population of adults with congenital heart disease (ACHD), among whom heart failure (HF) has emerged as a leading cause of morbidity and mortality. HF in ACHD represents a distinct and heterogeneous clinical entity shaped by lifelong abnormal loading conditions, prior surgical interventions, arrhythmogenic substrates, and limited representation in randomized clinical trials. The systemic ventricle may be morphologically left, right, or single, each conferring unique susceptibility to maladaptive remodeling, myocardial fibrosis, valvular dysfunction, and progressive contractile decline. Accurate diagnosis requires longitudinal, multimodal assessment incorporating echocardiography, cardiovascular magnetic resonance, cardiopulmonary exercise testing, biomarkers, rhythm surveillance, and selective invasive hemodynamic evaluation. Management prioritizes identification and correction of reversible contributors, including residual structural lesions, atrioventricular valve regurgitation, arrhythmias, pulmonary vascular disease, and extracardiac comorbidities. Pharmacologic therapy remains largely extrapolated from acquired HF paradigms and demonstrates variable efficacy across ACHD subgroups, underscoring the need for physiology-driven individualized care within specialized centers. Advanced therapies, including heart transplantation and mechanical circulatory support, are increasingly utilized, with improving outcomes despite higher perioperative complexity. This review presents a ventricle-based framework for understanding the pathophysiology, evaluation, and management of HF in ACHD and highlights critical gaps requiring further investigation.