There is evidence that multinutrient fortification of human milk increases in-hospital growth of preterm infants, but fortification has not been shown to improve long-term growth and neurodevelopmental outcome. We aimed to ascertain whether randomized controlled trials have determined the effect of early versus late introduction of fortifiers on growth and/or other outcomes, and have compared the efficacy/adverse effects of human milk-based versus cow milk-based fortifiers. We conclude that there is little evidence that early introduction of human milk fortification affects important outcomes, and limited evidence that a bovine fortifier places the infant at a higher risk of NEC.