Sudden unexpected infant death (SUID), also known as sudden unexpected death in infancy, or SUDI, is a term used to describe any sudden and unexpected death, whether explained or unexplained SUIDs, particularly those that occur during an unobserved sleep period (sleep-related infant deaths), such as unintentional suffocation, is challenging, cannot be determined by autopsy alone, and may remain unresolved after a full case investigation. Many of the modifiable and nonmodifiable risk factors for SIDS and suffocation are strikingly similar. This document focuses on the subset of SUIDs that occur during sleep.