The Naval Special Warfare Development Group, formerly known as SEAL Team SIX and based in Dam Neck, Virginia, is responsible for U.S. counterterrorist operations in the maritime environment. It's origin can be traced to the aftermath of the failed 1980 attempted to rescue American hostages at the Iranian Embassy (Operation Eagle Claw). Prior to this, the SEALs had already begun CT training, including all 12 platoons in SEAL Team One on the West Coast. On the East Coast, however, elements of the SEAL Team Two had taken the issue one step father. They formed a dedicated two-platoon group known as "MOB Six" (short for Mobility Six) in anticipation of a maritime scenario requiring a CT response and had begun training (including the development of advanced tactics such as "fast roping") to that end. Yet, as was the case with the US Army's initial CT unit - Blue Light - and Delta Force, only one group was needed and could be recognized as official. With the formal creation of SEAL Team Six (a name selected primarily to confuse Soviet intelligence as to the number of SEAL Teams in operation) in October 1980, MOB Six was demobilized. A large number of members, however, including the former MOB Six commander, were asked to join the fledgling group. With prior experience from these operators, aggressive leadership, and an accelerated training program, SEAL Team Six was declared mission-ready just six months later.

Training for Six was conducted throughout the United States and abroad, both on military and civilian facilities. Exchange programs and joint trainings were expanded with the more experienced international teams such as Germany's GSG-9, Great Britain's Special Boat Squadrons (SBS), and France's combat divers. In all cases, emphasis was placed on realism in training, in accordance with the "Train as you Fight, Fight as you Train" philosophy popular amongst most of the world's leading special operations and CT units.

Six participated in a number of operations, both overt and covert, throughout the 1980's (see list at the end of this section) before being revamped and renamed. The reasons for this transformation are vague, however the primary factor cited has been the need for the unit to evolved out of a poor reputation of the group within the Navy. A great deal of controversy was generated due to charges of misappropriation of funds and equipment by team members, as well as the conviction of unit founder Cdr. Richard Marcinko on charges of conspiracy, conflict of interest, making false claims against the government, and bribery. He was sentenced to nearly two years in a Federal penitentiary in addition to being forced to pay a $10,000 fine. Despite this turn of events, Marcinko is still revered in some SEAL circles as an almost mythical figure. This status was attained, in no small part, to a best selling-book series which centers around fictional maritime special operations and Counter Terrorism.

The US government has described the Naval Special Warfare Development Group as having been established to oversee development of NSW tactics, equipment, and techniques. This, of course, is only partly true. The unit is under the direct command of NAVSPECWARGRU, however it is also a component of Joint Special Operations Command (JSOC - Pope AFB, North Carolina), along with other CT units such as Delta Force and the 160th Special Operations Aviation Regiment (SOAR).

It is believed that DEVGRU maintains its own helicopter support unit (2 Sqns. with 18 HH-60H for SEAL transport and support), but trains frequently with the 160th SOAR, especially in support of ship assaults, which frequently make use of the small MH-6 "Little Bird", operated exclusively by the 160th. Organization and manpower of the Group is a classified, and can only be guessed at. It is estimated that NSWDG now numbers approximately 200 operators, broken down by teams, much like the British SAS and Delta Force. Most recently, it has been reported that there are currently four such teams within the group, assault units Red, Blue, Gold, and a special boat unit, Gray. The missions of these units are, again, a cause for speculation, however it is logical that they are specialized amongst themselves, perhaps along the lines of the SAS' Mountain, Mobility, Boat, and HALO troops (within a single Squadron). It is also possible that these units may have focus on specific target types instead, such as shipping, oil rigs, and structures, (although this scenario seems less likely due to the obvious need for all members of the Group to be current and proficient should a large scale operation arise). There is also an administrative and testing section, which numbers approximately 300 personnel. These individuals are responsible for the actual testing and development of new NAVSPECWAR equipment, including weapons. It has been reported that DEVGRU is one of only a handful of US units authorized to conduct preemptive actions against terrorists and terrorist facilities (NOTE: Red Cell once shared this charter, although it was never put into practice before the unit was disbanded). DEVGRU operators reportedly fire an average of 2,500 to 3,000 rounds per week in training.