| Abwehr | German military secret service, meaning ‘defence’ in German. |
| 30AU | 30 Assault Unit Royal Marines, also known as 30 Commando. |
| BNI | British Naval Intelligence. |
| ‘C’ | Head of British Secret Intelligence Service (SIS) MI6. |
| CIA | Central Intelligence Agency, replaced OSS, USA. |
| DEV | División Española de Voluntarios, Blue Division. |
| DNI | Director of Naval Intelligence, British. |
| Enigma | German code machine system. |
| Estado Novo | ‘New State’ Portuguese right-wing political party. |
| FET | Falange Española Tradicionalista, Fascist party of Spain. |
| FO | Foreign Office, British. |
| GCHQ | Government Communications Headquarters, Bletchley Park. |
| Gestapo | Geheime Staatspolizei, German secret state police. |
| GRU | Soviet military intelligence JIC Joint Intelligence Committee, British. |
| KO | Kriegs-Organisation stations Abwehr. |
| KTB | German National defence branch. |
| MoI | Ministry of Information, British. |
| MI5 | Military Intelligence section 5. British counter-intelligence service. |
| MI6 | Military Intelligence section 6. British espionage service, often known as SIS. |
| 17M | Signals section NID. |
| MEW | Ministry Economic Warfare, British. |
| NID | Naval Intelligence Division, British. |
| NKGB | The People’s Commissariat for State Security, Russian 1941–46. |
| NKVD | Narodnyi Komissariat Vnutrennikh Del, Soviet secret police. |
| NSA | National Security Agency, USA. |
| OKW | Oberkommando der Wehrmacht, German Armed Forces High Command. |
| OSS | Office of Strategic Services, replaced by CIA, USA. |
| PVDE | Portuguese Secret Police. |
| RM | Royal Marines. |
| RNVR | Royal Navy Volunteer Reserve. |
| RSHA | Reichssicherheitshauptamt, supreme state security department. Set up in 1939 to supervise the Gestapo and SD. |
| Section D | SIS sabotage section. |
| SD | Sicherheitsdienst, secret service branch of the SS. |
| SHAPE | Supreme Headquarters Allied Powers, Europe. |
| SIPO | Sicherheitspolizei, security police of the Gestapo and SD. |
| SIS | Secret Intelligence Service, British, also known as MI6. |
| SMERSH | ‘Death to Spies’, counter-intelligence service of the Red Army. |
| SOE | Special Operations Executive, British originally Section D of SIS. |
| SS | Schutzstaffel, or protection squad. The original title for Hitler’s bodyguard. |
| SO2 | Sabotage Section SOE (Naval). |
| Station X | GC & CS, Bletchley Park. |
| Ultra | Ultra decoded encrypted Axis radio communications classified ‘Ultra Secret.’ |
| W/T | Wireless Telegraph. |
| WTA | War Trade Agreement. |