Abstract Recent neurobiological and psychological research has established that vital development occurs in language, cognitive, motor and socio-emotional development during the first few years of life, and early life outcomes are key determinants of adult outcomes such as educational achievement, labor market outcomes, and health. Yet more than 200 million children under age five in LMICs will fail to reach their developmental potential as adults, predominantly due to poverty, poor health and nutrition, and inadequate cognitive and psychosocial stimulation. Early childhood development (ECD) interventions that integrate nutrition and child stimulation activities have been proposed as a powerful policy tool for the remediation of early disadvantages in poor settings, and numerous field studies have shown they can be effective in improving children's developmental and health outcomes, at least in the short-term. Yet key questions remain on what models of delivery are the most effective and cost-effective that can be potentially scalable in LMICs, as well as how to sustain parental behavioral changes over time, which can lead to long-term improvements in child development and the possibility of positive spillovers to benefit younger siblings. Having a better understanding of the underlying behavioral pathways leading from intervention, to parental behavior changes, to child impacts, is also key to inform policy about the optimal design of interventions to maximize their scalability and sustainability. Our study will conduct a multi-arm clustered randomized controlled trial across 60 villages and 1200 households in rural Kenya that tests different potentially cost-effective delivery models for an ECD intervention with a curriculum that integrates child psychosocial stimulation and nutrition education. Our hypothesis is that village-based group meetings that include a limited number of follow-up home visits to reinforce the messages and give greater opportunity to practice the new behaviors and receive personalized feedback is the most promising and cost-effective model able to achieve sustained impacts in parental behaviors and child outcomes. Selected households will undergo baseline and follow-up surveys to measure short-term impacts in parental behaviors and children's developmental outcomes, and we will collect data on potential mediators of parental behavioral change to uncover the pathways leading to impacts through a Decomposition Analysis. A follow-up survey two years after the end of the planned intervention will enable testing the sustainability of impacts, as well as the presence of any spillovers onto younger siblings. In collaboration with a local NGO, community health workers (CHWs) who sell health and nutrition products in their communities as a means to earn incomes will be trained to implement the intervention by integrating the ECD curriculum into their existing practices, significantly increasing the project's potential sustainabil...