Sudden infant death syndrome (SIDS) is the sudden unexpected death of a child younger than one year during sleep that cannot be explained after a postmortem evaluation including autopsy, a thorough history, and scene evaluation. The incidence of SIDS has decreased more than 50% in the past 20 years, largely as a result of the Back to Sleep campaign. The most important risk factors relate to the sleep environment. Prone and side sleeping positions are significantly more dangerous than the supine position. Bed sharing with a parent is strongly correlated with an increased risk of SIDS, especially in infants younger than 12 weeks. Apparent life-threatening events are not a risk factor for SIDS. Parents should place infants on their backs to sleep, should not share a bed, and should avoid exposing the infant to tobacco smoke. Other risk-reducing measures include using a firm crib mattress, breastfeeding, keeping vaccinations up to date, avoiding overheating due to overbundling, avoiding soft bedding, and considering the use of a pacifier during sleep once breastfeeding is established. One consequence of the Back to Sleep campaign is a significant increase in the incidence of occipital flattening. Infants who develop a flat spot should be placed with the head facing alternating directions each time he or she is put to bed. Supervised prone positioning while the infant is awake, avoiding excessive use of carriers, and upright positioning while awake are also recommended.