For the modern United States Special Forces unit see Delta Force.
Project DELTA was one of three Greek letter special forces reconnaissance projects formed by the Military Assistance Command, Vietnam, MACV during the Vietnam War to collect operational intelligence in remote areas of South Vietnam.
Project DELTA was established at Nha Trang in 1964 and consisted of six reconnaissance hunter-killer teams each composed of four United States Special Forces (USSF) and six to eight Vietnamese special forces (LLDB) with the 91st Ranger battalion. It was designated Detachment B-52, 5th Special Forces Group. 
DELTA's mission include operational and strategic reconnaissance into long held Vietcong areas and direct air strikes on them, they were also to conduct bomb damage assessment, conduct small scale reconnaissance and hunter-killer operations, capture and interrogate VC / NVA tap communications, bug compounds and offices, rescue downed aircrew and prisoners of war, emplace point minefields and other booby traps, conduct psychological operations, and perform counter intelligence operations. They were to focus on base areas and infiltration routes in the border areas.
DELTA originated as Project LEAPING LENA which was established on 15 May 1964 in order for the USSF to train LLDB teams for missions into Laos. The result of this was that five teams of eight men were parachuted into Laos and only five men survived the experience. The US did not participate. On 12 July 1964 a US Special Forces B-Team (B-110) and an A-Team (A-111) from the US 10th Special Forces Group arrived to take over the mission. By October 1964 Project LEAPING LENA was redesignated Project DELTA with USSF in control and serving alongside LLDB in actual operations. The main base was established at Nha Trang and half of the A-team at Dong Ba Thin to train the 91st Ranger Battalion.
DELTA ceased operations on 30 June 1970.
By 1966 DELTA consisted of the following:
Headquarters Section consisting of 31 US Special Forces and over 50 South Vietnamese Special Forces
Strike-Recondo Platoon(s) of 12 teams each with three US and three South Vietnamese special forces. Four more teams were added later.
Roadrunner Platoon consisting of six and then later twelve five man CIDG teams.
A Security Company consisting of 124 Nung.
Bomb Damage Assessment Platoon consisting of four US and 24 CIDG.
A 200 plus civilian work force was based at the Nha Trang basecamp and elements of it would accompany DELTA when it traveled to establish Forward Operating Bases (FOB).
MIKE Forces were battalion and then brigade size reaction forces assigned aid in case of hostile action but also by conducting economy of force operations. Project DELTA's MIKE Force was the 91st Ranger battalion.
Project Delta aviation support was provided by the 281st Assault Helicopter Company (10th Combat Aviation Battalion, 17th Combat Aviation Group, 1st Aviation Brigade), which was OPCON to 5th Special Forces Group in direct support of Project Delta operations.  The 281st AHC provided dedicated helicopter support from its arrival in Vietnam, 9 June 1966, until Project Delta was deactivated in June 1970. The 281st AHC "Intruders" are credited with developing special operations aviation tactics, as well as being the original special ops aviation unit. Forward air control support was originally supplied by pilots from the 19th Tactical Air Support Squadron; later the 21st Tactical Air Support Squadron assumed the mission.