Nutrition during infancy and early childhood is widely recognised as a crucial period in human development.(1) Commercial complementary foods are manufactured and marketed for young children to consume during the complementary feeding period and beyond as they transition to family foods.(2) These include products such as infant cereals, purees, squeeze pouches, ready-to-eat meals and snack foods. Recently, there has been a global increase in the availability and consumption of these foods, particularly foods in squeeze pouches. Parents use these products primarily for their convenience, perceived health benefits and affordability(3) However, accurately assessing the dietary intake of commercial complementary foods in infants and young children is uniquely challenging, compounded by the rapidly evolving commercial food market and their limited representation in food composition databases used by nutrition professionals to estimate dietary intake. Additionally, consolidated evidence on suitable dietary assessment methods or validated tools is limited. This study aimed to systematically review and synthesise the literature on how the dietary intake of commercial complementary foods is measured in infants and young children, with a focus on the methodologies applied. A systematic review was conducted using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines and registered with PROSPERO (ID: 1038587). A comprehensive search was conducted across five databases between April–May 2025, (Scopus, MEDLINE (EBSCO), PubMed (EBSCO), CINAHL and Web of Science). Study selection and data extraction were managed using Covidence. Two independent reviewers screened 2,307 studies, of which 81 were assessed at full text. Reference lists were manually reviewed, and a snowballing approach was employed to identify additional relevant studies. Results were summarised narratively. A total of fifty studies across twenty-six countries were included in this review. Twelve studies used a general questionnaire (e.g. basic questions such as: Has your child ever consumed commercial complementary foods?). Fourteen studies utilised a food frequency questionnaire to identify habitual intake of specific commercial complementary foods over a defined period (e.g. past week/month). Ten studies used 24-hour recalls, with half applying a multiple-pass method to capture detailed intake and portion sizes. Two studies applied a diet index that incorporated commercial complementary foods as part of a broader diet quality score. Twelve studies used dietary records, including four with weighed records that provided precise quantitative data on commercial complementary food consumption. This review highlights wide variation and key limitations in methods used to assess commercial complementary food intake for infants and young children. No validated or standardised tool currently exists, representing a major gap given the growing market and frequent use of these products. Future tools should be consumer-informed, easy to use, and validated for use in this population. Accurate measurement is essential for clinical care, surveillance, and understanding links to dietary patterns and health outcomes.