Nature, Published online: 29 January 2025; doi:10.1038/d41586-025-00193-1Sustainable engraftment of heart muscle cells (cardiomyocytes) can be achieved by implanting patches of engineered heart muscle onto the surface of the heart under immune suppression. Evidence from rhesus macaques and an individual with advanced heart failure suggests that structural and functional augmentation of the failing heart is possible.