Heart failure (HF) is a common cause of morbidity and mortality in congenital heart disease (CHD), with increasing prevalence because of improved treatment options and outcomes. Genetic factors and acquired postnatal factors in CHD might play a major role in the progression to HF. This article proposes 3 routes that lead to HF in CHD: rare monogenic entities that cause both CHD and HF; severe CHD lesions in which acquired hemodynamic effects of CHD or surgery result in HF; and, most commonly, a combined effect of complex genetics in overlapping pathways and acquired stressors caused by the primary lesion.