Early childhood is a critical period for development. The estimated costs of children not reaching their full developmental potential during this period are high and persist throughout life, including lower economic productivity and poorer health. Attaining full developmental potential has been recognized as the right of every child, and investing in early childhood development is key to breaking intergenerational cycles of poverty, increasing human capital, and improving population health and well-being. Yet the most recent estimates indicate that about 43% of children younger than 5 years in low- and middle-income countries remain at risk of poor development due to poverty and stunting alone. There is a pressing need for action to improve early childhood development in these settings, which must be informed by high-quality evidence from experimental studies on the most impactful interventions.