Over the last 20 years, the sudden infant death syndrome has become the leading cause of death in infants aged one month to one year in developed countries. The SIDS Referral Centers set up in France have been assigned the task of performing thorough clinical, metabolic, infectious and histologic studies. This post-mortem evaluation, whose results are difficult to interpret, is undertaken in an attempt to discriminate between the multiple causes of conditions present at the time of death. This classification task will improve the definition of a number of risk factors. Among these factors, prematurity, perinatal distress requiring resuscitation, and an unfavorable sociocultural environment are often mentioned. Other factors, including intrauterine growth retardation, dysmorphic disorders, impaired regulation of ventilation, heart rhythm anomalies, and inherited defects in fatty acid metabolism are still under study since they are all infrequent. Various combinations of these factors may result in increased vulnerability to stress during the first months of life, the period when SIDS is most common. This ongoing research is indispensable for providing advice and support of the family and developing appropriate individual preventive measures for newborn SIDS siblings.