When feeding at the breast is not possible, infants can still receive expressed mother’s own milk (MOM). Method of expression, hygiene practices and settings during expression, and processing can affect MOM composition. This study aimed to review current evidence on the influence of feeding MOM expressed using varying expression methods, hygiene practices or settings during expression, or treatments on the health and growth of recipient infants. We systematically searched CENTRAL, CINAHL, clinicaltrials.gov, Embase, Emcare, EU trials, Global Health, Global Index Medicus, MEDLINE, Scopus, Web of Science, and WHO for primary research studies, including observational studies, published up to March 2024 evaluating different methods of MOM expression, hygiene practices or settings during expression, and methods processing of MOM and reporting clinical outcomes on recipient infants. Key outcomes of interest were growth, mortality, morbidity, feeding tolerance, adverse events, cytomegalovirus (CMV) infection, retroviral infection, other infections, nutrient deficiencies, neurodevelopment, and breastfeeding. Qualitative thematic synthesis was conducted. An evidence gap map was produced using the Grading of Recommendations, Assessment, Development, and Evaluation approach. Of 29,320 studies screened, 45 met the inclusion criteria. No expression method or pump type showed clear benefits for breastfeeding rates or infant growth. Three studies reported improved weight gain in infants receiving hindmilk. Evidence on the effect of processing methods on morbidity and mortality was inconclusive. Limited evidence was found on the efficacy of the freeze-thaw cycle in reducing CMV transmission, whereas pasteurization proved more effective. No studies assessed clinical outcomes related to hygiene practices or expression settings. The use of hindmilk improves infant weight gain with some certainty of evidence. Hand expression of MOM has similar efficacy to that of electric pumping on the growth of recipient infants, including preterm infants. Evidence on clinical outcomes of different MOM expression practices and treatments is very limited. This work underscores the need for future studies to address the substantial evidence gaps identified.