Growth monitoring and promotion is an integral part of preventive and continuum of care for newborns and children. It is feasible, easy to conduct, and can be readily integrated into routine practice. Regular growth monitoring is an essential screening tool for early identification of potential growth problems that can trigger the need for further evaluation or the provision of appropriate interventions. These interventions include counselling on responsive care and infant and young child feeding practices as well as treatment for acute malnutrition. Tracking child growth in terms of attained size using international standards enables practitioners to detect when a child's growth deviates substantially from expected patterns at specific ages and to monitor growth over time. Hence, it is critical that international standards serve as a gold standard, reflecting growth patterns among healthy children with adequate nutrition and who live in optimal environmental conditions. Currently, there are two complementary international growth standards for monitoring children under 5 years of age—the World Health Organization Child Growth Standards (WHO GS) 1 and the INTERGROWTH-21st Standards (IG-21st) 2-4. This supplement includes four interlinked articles from the Guidance for International Growth Standards (GIGS) initiative 5. The aim of the initiative is four-fold: To provide evidence on differences observed when national and international standards are applied to the same population; to highlight the risks of misclassifying children as growing poorly or growing well when growth standards are inappropriately applied; to provide a suite of statistical tools to support the correct application of the two international growth standards; and to make available clear guidance on which growth standards to use, for whom, and when. The four articles in this supplement, building on previous work 6, 7, present empirical evidence on the magnitude of misclassification that can occur when international standards are not used or when the two international standards are used inappropriately. They also stress that the inconsistent application of international standards and the wide variation of national growth charts undermine the ability to collect comparable data. Lack of robust, comparable data hampers the ability to reliably track progress in addressing poor child growth patterns across contexts and time, which limits accountability efforts and hinders the development and provision of effective interventions. The articles note that these problems are particularly severe for small vulnerable newborns (small for gestational age, low birthweight, and preterm babies). The GIGS initiative and these four articles are timely given increased focus in recent years on saving the lives of small vulnerable newborns 7, 8 and a renewed emphasis on scaling up integrated, high impact health and nutrition services for children, including growth monitoring and promotion 9. Although there has been significant progress in reducing global child mortality rates in the past twenty years, the rate of progress has slowed since the start of the Sustainable Development Goal era, particularly for neonatal mortality 10. Recent evidence similarly shows that global low birthweight and preterm birth rates have changed little in the past few decades 11, 12, and preterm birth remains a leading cause of death in children under the age of five 10, 11. Both these conditions are associated with higher risks of poor health and development in the short and long term, with implications for human capital development. The latest global nutrition estimates also show a stark picture of little improvement in stunting prevalence in the past 20 years; and there are mounting concerns that headwinds such as increased numbers of conflict situations, climate-related disasters, and countries experiencing political and economic instability will place children at higher risk of wasting, a life-threatening form of acute malnutrition 13, 14. However, as these four papers point out, the correct application of available international growth standards is important to correctly identify babies and children most at risk of suboptimal growth and to provide them with the nurturing care they need to survive and thrive. Each of the four papers contributes new evidence, summarised below, supporting the need for greater adoption of the two international standards—IG-21st and WHO GS—for regular assessments of newborn and child growth. They also present a suite of statistical packages generated by the GIGS project as global goods that provide user-friendly, structured, operational guidance on consistent and appropriate application of the two standards. The first paper in the supplement from Ngatia et al. is a systematic review of 76 studies across 38 countries published between 2007 and 2023 15. The review identifies national and international child growth charts currently in use and how they compare. The authors found that international standards frequently identify a higher prevalence of stunting and child underweight compared to national charts, underscoring the need for harmonisation of growth charts to ensure accurate, reliable child growth assessments and to best inform clinical practice. The second paper by Ohuma et al. 16 recounts the historical development of growth charts and compares available descriptive, prescriptive, and customised charts. The authors explain, based on their comparative analysis, why it is critical to use the two available international standards so that newborns and children are correctly assessed and classified as growing well or poorly. They also stress the need for clear, operational guidance on how best to apply them. The GIGS statistical tools incorporate the two international standards by providing this user-friendly, practical guidance and supporting the consistent application of the international standards across settings and time. In the third paper, Vesel et al. 17 use data from the Low Birthweight Infant Feeding Exploration study 18 in Malawi and Tanzania to compare growth assessment values using the WHO GS and IG-21st standards. They show that using WHO GS alone tends to result in the misclassification of preterm babies due to a misalignment of the postnatal and postmenstrual age scales. In contrast, IG-21st provides gestational age–specific evaluation (i.e., using the postmenstrual age-scale) that is better aligned with expected growth of babies born preterm. They recommend that the two classifications are used together as appropriate. For example, for preterm babies, the IG-21st standards should be applied during the first six months of life for accurate growth monitoring and the WHO GS applied thereafter. The last article in the supplement by Ohuma et al. 19 presents a scenario-driven approach to illustrate and show the considerable variations in growth assessment depending upon which standard is applied for term and preterm babies. Consequently, the paper provides clear guidance on when, for whom, and how to apply the WHO GS and IG-21st standards. These four articles, viewed together, make an evidence-based case for why there should be greater uptake of the WHO GS and IG-21st standards and how the GIGS suite of statistical tools can practically support this uptake. The evidence is unmistakable that the use of non-standard charts and inappropriate application of the international standards can lead to substantial misclassification in the evaluation of newborn and child growth. This can result in unnecessary care for children misclassified as growing poorly and missed care for children who are misclassified as growing well. Consistent application of the two international standards can also improve comparability of growth assessment results across contexts and time, which is important for informing policy and programmes to improve newborn care and child growth. Correct use of the standards can also filter into better national and global estimates of stunting, wasting and other growth problems in children and newborns that can strengthen accountability, resource mobilisation, and research efforts. In addition, GIGS provides a unified framework for tracking progress made towards international child growth and nutrition targets such as the global nutrition targets 2030 20 and Sustainable Development Goal 2.0 21. The evidence generated through the GIGS initiative shows the many benefits of adoption of the international standards for growth monitoring and promotion, a cornerstone of quality paediatric care. The GIGS set of practical tools can help clinicians and researchers at the local and national level transition from reliance on local reference charts to consistent application of the two international standards. A shift to widescale use of the two growth standards will require countries to adopt and implement appropriate policies, including adequately training providers and strengthening supply chains to ensure all facilities are equipped with necessary growth monitoring materials. Broad adoption of the standards could ultimately contribute to faster progress towards global and national goals for improving child and newborn health and wellbeing, a lofty aim worth striving for. JR and KM led the development of the first draft and all revisions of the manuscript. NP, MW, EB, FM, JL, KS, and QX provided substantial inputs into all drafts and reviewed and approved the final version. We thank Professor Eric Ohuma (London School of Hygiene & Tropical Medicine, UK) and Dr. Linda Vesel (Ariadne Labs, Harvard T.H. Chan School of Public Health/Brigham and Women's Hospital, Boston, Massachusetts, USA), the co-leads for the Guidance for International Growth Standards (GIGS) project, for their guidance and input in writing this commentary. We would also like to thank the GIGS Technical Advisory Group members for their contributions. The GIGS Technical Advisory Group is composed of growth and nutrition experts from around the world with expertise in policy, research, program implementation, and paediatrics who were brought together to advise on key outputs throughout the GIGS project and will support dissemination and uptake of the GIGS tools. The authors have nothing to report. The authors have nothing to report. The authors declare no conflicts of interest. The authors have nothing to report. Appendix S1: Technical Advisory Group (TAG) Members and Affiliations. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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Requejo et al. (Mon,) studied this question.