WHO has previously published three reports related to child development. The first, A critical link (WHO, 1999) summarized the importance of addressing both the nutritional and psychosocial aspects of malnutrition and was a seminal report on the need for developmentally based biopsychosocial approaches to child health. The second, The importance of caregiver–child interactions for the survival and healthy development of young children (Richter, 2004) contained important information on the most critical component of child development, the relational aspect. The third, Early childhood development: a powerful equalizer (Irwin, Siddiqi & Hertzman, 2007), prepared for the WHO Commission on the Social Determinants of Health, provided an in-depth look at the importance of social determinants in shaping child development This fourth report includes information from low- and middle-income countries on the conceptualization, epidemiology, prevention, detection, assessment and early management of the broad spectrum of developmental risk factors and developmental difficulties in children aged 3 years and under. It does not contain in-depth information on specific developmental risk factors, such as human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS), low birth weight, malnutrition and chronic illness. This review should not be viewed as a general resource for early childhood disability or for specific developmental disabilities, such as cerebral palsy, genetic or metabolic disorders, autism spectrum disorders, and cognitive or sensory impairments. These topics deserve specific attention beyond the scope of this review. Where relevent and possible, reference is made to other reviews or documents on these topics.