PROJECT SUMMARY More than 40% of children under 5 years in low- and middle-income countries (LMICs) are at risk of not reaching their developmental potential. One of the most effective and proven strategies for supporting early child development in these settings is to empower caregivers and communities to support their children’s development. However, evidence on the effectiveness and the barriers and facilitators to implementation of interventions to promote this nurturing care are need. In this study, we will use an implementation science framework to evaluate an individualize intervention to promote nurturing care—the international Guide for Monitoring Child Development (GMCD), in rural India and Guatemala. We will conduct a hybrid effectiveness/implementation cluster randomized trial, where children aged 0-2 years will receive individualized visits from frontline health workers administering the GMCD. The study will have three parts: In Part 1, we will randomize clusters to receive either the GMCD or usual care for 12 months. After 12 months, control clusters will cross-in and all children will receive an additional 12 months of intervention. We will assess the impact of the intervention at 12 and 24 months on developmental outcomes and on the home care environment. In Part 2, we will use the RE-AIM implementation science framework to assess the Reach, Effectiveness, Adoption, Implementation, Maintenance of the intervention. We will also use the CFIR (Consolidated Framework for Implementation Research) framework to conduct in-depth complementary qualitative evaluation of implementation barriers and facilitators in high- and low- performing clusters from Part 1. In Part 3, we will assess cost effectiveness of the intervention. In conclusion, the study will answer three important questions: (1) Is the GMCD effective at improving developmental outcomes and the home care environment for children at risk in rural India and Guatemala? (2) What real-world institutional and contextual factors influence the impact of the intervention and might affect its potential ability to sustainably reach children and families? (3) Is the intervention cost-effective? The project will generate globally relevant evidence on community-based early child development interventions in LMICs.