Breast milk is the best source of nourishment for both full-term and preterm newborns. However, in preterm newborns, exclusive breastfeeding can lead to nutritional deficiencies, with short- and long-term consequences on growth and neurocognitive development. Breast milk fortification is a widely used strategy to provide an adequate nutritional profile to these patients. There is currently a great heterogeneity in terms of fortification patterns and the type of fortifier. The present systematic review summarizes the current scientific evidence to evaluate the different strategies for fortification of human milk in preterm infants, their possible differences on benefits, and the hypothetical impact on prematurity outcomes.