Preterm infants fed fortified human milk (HM) in standard (STD) fashion grow slower than preterm formula fed infants. Recently, low protein intake has been proven to be the primary limiting factor responsible for this growth failure. The main reason of protein undernutrition despite fortification is that STD fortification is based on the customary assumptions about the composition of HM. However, the protein concentration of preterm HM is variable and decreases with the duration of lactation. Also, the protein concentration of banked donor milk, which is most often provided by mothers of term infants, is likely to be lower. Hence, most of the HM fed to preterm infants during the fortification period is likely to have an inadequately low protein concentration. This hypothesis has been confirmed very recently by comparing the assumed and actual protein intakes in preterm infants fed fortified HM. Novel fortification models have been devised to deal with the problem of ongoing protein undernutrition. Individualized fortification is the recommended method to optimize HM fortification. There are two models of individualization: "adjustable fortification" and "targeted fortification". Both ways are feasible and effective in improving protein intakes and growth. Adjustable fortification has the advantage of being practical and avoids excessive protein intakes.