Sudden infant death syndrome (SIDS), sometimes known as cot death or crib death, is the sudden unexplained death of a child of less than one year of age. Diagnosis requires that the death remain unexplained even after a thorough autopsy and detailed death scene investigation. SIDS usually occurs between the hours of midnight and 9:00 a.m., or when the baby is sleeping. There is usually no noise or evidence of struggle. SIDS remains one of the leading causes of infant mortality in Western countries, constituting almost 1/3 of all post-neonatal deaths.

The exact cause of SIDS is unknown. The requirement of a combination of factors including a specific underlying susceptibility, a specific time in development, and an environmental stressor has been proposed. These environmental stressors may include sleeping on the stomach or side, overheating, and exposure to tobacco smoke. Accidental suffocation from bed sharing (also known as co-sleeping) or soft objects may also play a role. Another risk factor is being born before 37 weeks of gestation. Between 1% and 5% of SIDS cases are estimated to be misidentified infanticides caused by intentional suffocation. SIDS makes up about 80% of sudden and unexpected infant deaths (SUIDs). The other 20% of cases are often caused by infections, genetic disorders, and heart problems.

The most effective method of reducing the risk of SIDS is putting a child less than one-year-old on their back to sleep. Other measures include a firm mattress separate from but close to caregivers, no loose bedding, a relatively cool sleeping environment, using a pacifier, and avoiding exposure to tobacco smoke. Breastfeeding and immunization may also be preventative. Measures not shown to be useful include positioning devices and baby monitors. Evidence is not sufficient for the use of fans. Grief support for families affected by SIDS is important, as the death of the infant is unexpected, unexplained, and can cause suspicion that the infant may have been intentionally harmed.

Rates of SIDS vary nearly tenfold in developed countries from one in a thousand to one in ten thousand. Globally, it resulted in about 19,200 deaths in 2015, down from 22,000 deaths in 1990. SIDS was the third leading cause of death in children less than one year old in the United States in 2011. It is the most common cause of death between one month and one year of age. About 90% of cases happen before six months of age, with it being most frequent between two months and four months of age. It is more common in boys than girls. Rates of SIDS have decreased by up to 80% in areas with "Safe to Sleep" campaigns.

The syndrome applies only to infants under one year of age. SIDS is a diagnosis of exclusion and should be applied to only those cases in which an infant's death is sudden and unexpected, and remains unexplained after the performance of an adequate postmortem investigation, including:

After investigation, some of these infant deaths are found to be caused by suffocation, hyperthermia or hypothermia, neglect or some other defined cause.

Australia and New Zealand shifted to sudden unexpected death in infancy (SUDI) for professional, scientific, and coronial clarity:

The term SUDI is now often used instead of sudden infant death syndrome (SIDS) because some coroners prefer to use the term 'undetermined' for a death previously considered to be SIDS. This change is causing a diagnostic shift in the mortality data.