A special mission unit (SMU), at one time referred to as a "tier 1" unit,[1] is a designation for the United States military's most highly secretive and elite special operations forces.[2] The term special missions unit is also used in Australia to describe the Special Air Service Regiment. Special mission units have been involved in high-profile military operations, such as the killing of Osama bin Laden and the attempted hostage rescue of Kayla Mueller.