Children's experiences during early childhood are critical for their cognitive and socioemotional development, two key dimensions of human capital. However, children from low-income backgrounds often grow up lacking stimulation and basic investments, which leads to developmental deficits that are difficult, if not impossible, to reverse later in life without intervention. The existence of these deficits is a key driver of inequality and contributes to the intergenerational transmission of poverty. In this article, we discuss the framework used in economics to model parental investments and early childhood development and use it as an organizing tool to review some of the empirical evidence on early childhood research. We then present results from various important early childhoods interventions, with an emphasis on developing countries. Bringing these elements together, we draw conclusions on what we have learned and provide some directions for future research.