Sudden infant death syndrome (SIDS) is the death of a baby who is younger than 1 year old without a known cause. Typically, a parent or other caregiver puts the baby—who seems healthy—down to sleep and returns later to find the baby has died.

No one is at fault when a baby dies of SIDS: it can be neither predicted nor completely prevented. A baby's death is not considered a case of SIDS when a specific cause is discovered, such as carbon monoxide poisoning. By definition, SIDS is considered the cause of a baby's death only when the death remains unexplained, even after a thorough investigation.

SIDS is also known as sudden unexpected infant death (SUID).

Placing babies on their backs when putting them down to sleep reduces the risk of SIDS.