Abstract An estimated 43% of children under age 5 in low- and middle-income countries (LMICs) experience compromised development due to poverty, poor nutrition, and inadequate psychosocial stimulation. Numerous early childhood development (ECD) parenting interventions have been shown to be effective at improving ECD outcomes, at least in the short-term, but they are a) still too expensive to implement at scale in low-resource and rural settings, and b) their early impacts tend to fade over time in the absence of continued support. New ways to deliver effective ECD parenting interventions are sorely needed that are both low-cost to be potentially scalable, while also able to sustain impacts long-term. The rapid growth and low cost of mobile communications in LMIC settings presents a potentially promising solution to the competing problems of scalability and sustainability. Yet there is no rigorous research on mobile-health (mHealth) interventions for ECD outcomes in LMIC settings. We recently showed that an 8- month ECD parenting intervention featuring fortnightly group meetings delivered by Community Health Workers (CHWs) from Kenya's rural health care system significantly improved child cognitive, language, and socioemotional development as well as parenting practices, and our group-based delivery model was more cost-effective than previous ECD interventions. Yet it is still too expensive for scaling in a rural LMIC setting such as ours, particularly if we need interventions that can be extended for longer periods of time to increase their ability to sustain impacts. Our proposed study experimentally tests a traditional in-person delivery model for an ECD parenting intervention against two mHealth-based delivery models that partially or almost fully substitute remote delivery for in-person meetings. Kenya is an ideal setting for testing mHealth programs given its high penetration of mobile phones (94%). We will assess the relative effectiveness and costs of these mHealth delivery models against a purely in-person model, and extend the interventions over two years to increase their ability to sustain changes in child outcomes longer term. Our evaluation design is a non- inferiority clustered Randomized Control Trial across 60 CHWs and 1200 households in which we will use an adaptive trial design to allow for midcourse review and feedback on the remote delivery models. By testing three interventions that vary in how much in-person delivery is substituted by remote-delivery, we can assess the degree of substitutability or complementarity to inform the design of more scalable and sustainable interventions. Our goal is to determine the best model to maximize the intervention's reach and sustained impacts to improve child outcomes. By integrating delivery into the ongoing operations of local CHWs within Kenya's rural health care system, utilizing new low-cost technology, and involving local ECD policymakers and stakeholders as key collaborators from t...