One out of every three children under age 5 in developing countries lives in conditions that impede human capital development. In this study, we survey the literature on parenting training programs implemented before age 5, with the aim to increase parental investment in human capital accumulation in developing countries. Our review focuses on the implementation and effectiveness of parenting training programs (i.e., training in child psychosocial stimulation and/or training about nutrition). We emphasize the mechanisms that drive treatment-induced change in human capital outcomes and identify the demand- and supply-side behaviors that affect efficacy and effectiveness. Although the literature includes evidence on program features that are associated with successful interventions, further evidence on the dynamics of human capital formation, documentation of medium- to long-term persistence of treatment impacts, and research on the implementation and evaluation of programs at scale are needed to delineate a scalable and inclusive program that provides long-term treatment impacts. Expected final online publication date for the Annual Review of Resource Economics, Volume 14 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.