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't be predicted or completely prevented. A baby's death is not considered a case of SIDS when a specific cause is found, such as carbon monoxide poisoning. SIDS is considered the cause of a baby's death only when the death remains unexplained, even after a thorough review.

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

Until the first birthday, placing babies on their backs when putting them down to sleep reduces the risk of SIDS.