Infants dying of sudden infant death syndrome (SIDS) constitute the single largest component of postneonatal infant mortality. Many different scientific approaches and hypotheses have been pursued in research studies over the past two decades. The field of epidemiology provides a comprehensive view of risk factors for SIDS with which theories of underlying physiologic defects or other putative causes should conform. Epidemiologic research has a vital role to play in assessing hypotheses derived from other types of studies.