BackgroundThe first 5 years of life are crucial for optimising growth, health, and cognitive development. However, many children do not reach their full cognitive and developmental potential because of multilevel barriers, including those resulting from poverty and homelessness. This review aims to summarise the evidence characterising the barriers to optimal health and cognitive outcomes and to accessing health services, focusing on homeless children younger than 5 years of age (under 5s), which is one of the most vulnerable populations. MethodsFor this narrative review, we searched Medline, PubMed, Embase, CINAHL, Web of Science, OVID Maternity and Infant Care, and The Cochrane Library (publications dates from Jan 1, 1980, to Aug 5, 2019) without language restrictions using the key words and MESH headings “homelessness”, “housing”, “paediatrics”, “interpersonal relations”, “social exclusion”, “toddler”, “children under 5”, “engagement”, and “communication and insecurity”. The search strategy yielded 1663 articles. Retrieved articles were organised by study design. Because of the considerable heterogeneity of methods and outcomes, we used a narrative synthesis approach following guidelines by the University of York Centre for Reviews and Dissemination. The primary outcome was barriers to optimal health and accessing health services, focusing on under 5s. Findings24 full texts were selected in the final synthesis, including primary research studies and systematic or narrative reviews from high-income countries. There was little evidence describing links among housing insecurity, health, and cognitive outcomes in under 5s. This age group was rarely studied as a discrete group and often combined with older ages (eg, ≤24 years). The quality of articles varied greatly because of the heterogeneity in study design. Nevertheless, we identified important themes: barriers were described at the individual and family level (eg, race or ethnicity, immigration status, and fear), system level (eg, poor access to medication, absence of care plan, and no insurance) and community level (eg, transportation limitations and poor housing conditions). InterpretationMore methodologically rigorous research is needed to identify what barriers exist for under 5s and their parents in accessing health services, and how this affects the child's health. Such data is needed to promote the co-production of cross-sector tailored interventions to address these barriers by working directly with experts by experience of social exclusion and their children. FundingNone.