Developmental disabilities include limitations in function and activities resulting from disorders of the developing nervous system in conjunction with unaccommodating environments or absence of assistive technologies. This chapter discusses key principles and considerations in designing and implementing screening programs and epidemiologic studies of developmental disabilities. The epidemiologic studies in low- and middle-income countries were frequently conducted by foreign researchers and sometimes characterized as "helicopter epidemiology". One of the challenges of epidemiologic studies of disability is that case status is often based on information obtained from questionnaires or cognitive tests that are designed and validated for use in one language and culture, and may not be applicable for or capable of generating comparable data across cultures. It is likely that for the majority of the world's children with developmental disabilities, obtaining an accurate diagnosis, though an important step, comes with no guarantee that coordinated services and appropriate services will be available.