Mother's own milk is universally recognized as the optimal source of nutrition for preterm infants, although most authorities agree a multi-nutrient fortifier must be added in order to support nutrient accretion at a rate comparable to in utero. Nevertheless, many preterm infants face a gap between achieved growth and what could have been achieved in utero. In this narrative review, we provide an overview on the macronutrient content in mother's own milk and donor milk and how this can be enhanced by the various available multi-nutrient fortifiers. We describe their general compositions and formulation, as well as several of their theoretical and practical advantages and drawbacks. In addition, differences between standardized fortification, or a more individualized approach like adjusted and targeted fortification are discussed. The optimal strategy however remains to be elucidated, and more experimental well-powered studies are therefore urgently needed. Until then, financial considerations and practical capabilities are likely to be the main drivers of local fortification strategies.