Human milk is the feed of choice for preterm infants. However, human milk does not provide enough nutrition, especially protein, for preterm infants to achieve target growth rates similar to those in utero (15-20 g/kg per day). Fortifiers for human milk, manufactured from bovine milk, are commercially available and routinely used for patients born <32 weeks' gestation prior to discharge home. Recent recommended dietary intakes (RDI) have been revised. Up to 4.2 g of protein and 135 kcal/kg per day is recommended for infants born very preterm. Additional supplements are needed to current commercial fortifiers to achieve these RDI and reduce the incidence of ex-uterine growth failure. A human milk fortifier that is manufactured from donor human milk is available in some developed countries and may confer some clinical benefits, including a reduction in necrotizing enterocolitis. Fortification can be added in a standardized protocol as per manufacturers' instructions. Human milk composition can be analyzed and fortification individualized to take into account the large variation from mother to mother. Alternatively, fortification can be increased in a stepwise manner based on assumed composition while monitoring blood urea levels for safety. The current aim is to prevent preterm infants dropping percentiles and falling below the 10th percentile at 36 weeks' corrected gestational age or discharge home. More data are required on how best to fortify human milk for preterm infants to achieve optimal growth, development and health outcomes in the long term. There is an urgent need for well-designed and informed randomized clinical trials in this vulnerable preterm population.