An estimated 250 million children in low- and middle-income countries are unable to meet key developmental milestones due to biological, psychological, and social risk factors, including maternal and child malnutrition, maternal depression, and inadequate early stimulation opportunities. Without early intervention, these adversities pose threats to longer-term development, including poor adult physical and mental health, lower educational attainment, and low income. Given these complex risks, nutrition inputs alone are unlikely to be sufficient to optimize early childhood development, and bundling with early stimulation and responsive care inputs has been proposed. A robust body of evidence now shows that early stimulation and responsive care interventions in the first 1000 days of life promote early development. However, studies of bundled or integrated early childhood stimulation and nutrition interventions on growth and development outcomes have yielded mixed results, and more investigations are needed.