Special forces
Training continued by day and night with river crossings, mountain climbing, weapons training, unarmed combat, map reading, and small boat operations on the syllabus.
Reaching a wartime strength of over 30 individual units and four assault brigades, the Commandos served in all theatres of war from the Arctic Circle to Europe and from the Mediterranean and Middle East to South-East Asia. Their operations ranged from small groups of men landing from the sea or by parachute to a brigade of assault troops spearheading the Allied invasions of Europe and Asia. The first modern special forces units were established by men who had served with the Commandos, including the Parachute Regiment, Special Air Service, and Special Boat Service. The No. 10 (Inter-Allied) Commando organised by British of volunteers from occupied Europe led to French Commandos Marine, Dutch Korps Commandotroepen, Belgian Paracommando Brigade.

Special Air Service (SAS): The first modern special forces unit was the Special Air Service (SAS), formed in July 1941 from an unorthodox idea and plan by Lieutenant David Stirling. In June 1940 he volunteered for the No. 8 (Guards) Commando (later named "Layforce"). After Layforce was disbanded, Stirling remained convinced that due to the mechanized nature of war a small team of highly trained soldiers with the advantage of surprise could exact greater damage to the enemy's ability to fight than an entire platoon. His idea was for small teams of parachute trained soldiers to operate behind enemy lines to gain intelligence, destroy enemy aircraft, and attack their supply and reinforcement routes. Following a meeting with the C-in-C Middle East, General Claude Auchinleck, his plan was endorsed by the Army High Command.

The force initially consisted of five officers and 60 other ranks. Following extensive training at Kabrit camp, by the River Nile, L Detachment, SAS Brigade, undertook its first operations in the Western Desert. Stirling's vision was eventually vindicated after a series of successful operations. In 1942, the SAS attacked Bouerat. Transported by the Long Range Desert Group (which carried out deep penetration, covert reconnaissance patrols, intelligence missions and attacks behind the enemy lines from 1940), they caused severe damage to the harbour, petrol tanks and storage facilities. This was followed up in March by a raid on Benghazi harbour with limited success but they did damage to 15 aircraft at Al-Berka. The June 1942 Crete airfield raids at Heraklion, Kasteli, Tympaki and Maleme significant damage was caused, and raids at Fuka and Mersa Matruh airfields destroyed 30 aircraft.

Chindits: In the Burma Campaign, the Chindits, whose long-range penetration groups were trained to operate from bases deep behind Japanese lines, contained commandos (King's Regiment (Liverpool), 142 Commando Company) and Gurkhas. Their jungle expertise, which would play an important part in many British special forces operations post-war, was learned at a great cost in lives in the jungles of Burma fighting the Japanese.

The Company of Chosen Immortals: Immediately after the German occupation of Greece in April–May 1941, the Greek government fled to Egypt and started to form military units in exile. Air Force Lt. Colonel G. Alexandris suggested the creation of an Army unit along the lines of the British SAS. In August 1942 the Company of Chosen Immortals (Greek: Λόχος Επιλέκτων Αθανάτων) was formed under Cavalry Major Antonios Stefanakis in Palestine, with 200 men. In 1942, the unit was renamed Sacred Band. In close cooperation with the commander of the British SAS Regiment, Lt. Colonel David Stirling, the company moved to the SAS base at Qabrit in Egypt to begin its training in its new role. The special forces unit fought alongside the SAS in the Western Desert and the Aegean.

Poland: During the start of World War II “September campaign,” the Polish Government did not sign the capitulation, but moved to Paris and then to London. In an attempt to achieve its aims the government in exile gave orders to the Polish resistance and formed a special military unit in Britain with the soldiers called Cichociemni (“silent and unseen”) paratroopers to be deployed into Poland. The Cichociemni were trained similar to the British Special Forces, with the curricula differing according to each soldier's specialization. Their task, on deployment to Poland, was to sustain the structures of the Polish state, training the members of the Resistance in fighting the German occupant. This included taking part in the Warsaw Uprising.

Australian: Following advice from the British, Australia began raising special forces. The first units to be formed were independent companies, which began training at Wilson's Promontory in Victoria in early 1941 under the tutelage of British instructors. With an establishment of 17 officers and 256 men, the independent companies were trained as "stay behind" forces, a role that they were later employed in against the Japanese in the South West Pacific Area during 1942–43, most notably fighting a guerrilla campaign in Timor, as well as actions in New Guinea. In all, a total of eight independent companies were raised before they were re-organised in mid-1943 into commando squadrons and placed under the command of the divisional cavalry regiments that were re-designated as cavalry commando regiments. As a part of this structure, a total of 11 commando squadrons were raised.
They continued to act independently and were often assigned at brigade level during the later stages of the war, taking part in the fighting in New Guinea, Bougainville and Borneo, where they were employed largely in long-range reconnaissance and flank protection roles. In addition to these units, the Australians also raised the Z Special Unit and M Special Unit. M Special Unit was largely employed in an intelligence-gathering role, while Z Special Force undertook direct action missions. One of its most notable actions came as part of Operation Jaywick, in which several Japanese ships were sunk in Singapore Harbour in 1943. A second raid on Singapore in 1944, known as Operation Rimau, was unsuccessful.

United States: 

Office of Strategic Services: The United States formed the Office of Strategic Services (OSS) during World War II under the Medal of Honor recipient William J. Donovan.  This organization was the predecessor of the Central Intelligence Agency (CIA) and was responsible for both intelligence and special forces missions. The CIA's elite Special Activities Division is the direct descendant of the OSS.

Marine Raiders: On February 16, 1942, the U.S. Marine Corps activated a battalion of Marines with the specific purpose of securing beach heads, and other special operations. The battalion became the first modern special operations force of the U.S. The battalion became known as Marine Raiders due to Admiral Chester Nimitz's request for "raiders" in the Pacific front of the war.

United States Army Rangers: The history of the United States Army Rangers specialist soldier dates back to the 17th through 19th century from military units such as United States Mounted Rangers, United States Rangers and Texas Rangers. In WWII mid-1942, Major-General Lucian Truscott of the U.S. Army, a General Staff submitted a proposal to General George Marshall onceived under the guidance of then Army Chief of Staff, General George C. Marshall, that selectively trained Ranger soldiers were recruited for the newly established special operations Army Ranger Battalion.

1st Special Service Force: The United States and Canada formed the 1st Special Service Force as a sabotage ski brigade for operations in Norway. Later known as the "Devil's Brigade" (and called "The Black Devils" by mystified German soldiers), the First Special Service Force was dispatched to the occupied Aleutian Islands, Italy and Southern France.

Merrill's Marauders: Merrill's Marauders were modeled on the Chindits and took part in similar operations in Burma. In late November 1943, the Alamo Scouts (Sixth Army Special Reconnaissance Unit) were formed to conduct reconnaissance and raider work in the Southwest Pacific Theater under the personal command of then Lt. General Walter Krueger, Commanding General, Sixth U.S. Army. Krueger envisioned that the Alamo Scouts, consisting of small teams of highly trained volunteers, would operate deep behind enemy lines to provide intelligence-gathering and tactical reconnaissance in advance of Sixth U.S. Army landing operations.

Special Forces Tab: In 1983, nearly 40 years after the end of World War II, the US Army created the Special Forces Tab.  It was later decided that personnel with at least 120 days' wartime service prior to 1955 in certain units, including the Devil's Brigade, the Alamo Scouts and the OSS Operational Groups, would receive the Tab for their services in World War II, placing them all in the lineage of today's U.S. and Canadian (via Devil's Brigade) Special Forces.

Axis powers: The Axis powers did not adopt the use of special forces on the same scale as the British.

German: The German army's Brandenburger Regiment was founded as a special forces unit used by the Abwehr for infiltration and long distance reconnaissance in Fall Weiss of 1939 and the Fall Gelb and Barbarossa campaigns of 1940 and 1941.