German electrical equipment manufacturer formed in 1966 through the merger of Siemens & Halske AG (founded 1847), Siemens-Schuckertwerke (founded 1903), and Siemens-Reiniger-Werke AG (founded 1932). Operating in more than 190 countries, it engages in a wide range of manufacturing and services in areas such as power generation and transmission, transportation, lighting, electrical components, telecommunications systems, and medical engineering. The company invests heavily in research and development and ranks among the largest patent holders in the world. Headquarters are in Munich.

The first Siemens company, Telegraphen-Bau-Anstalt von Siemens & Halske (“Telegraph Construction Firm of Siemens & Halske”), was founded in Berlin in 1847 by Werner Siemens (Siemens, Werner von) (1816–92), his cousin Johann Georg Siemens (1805–79), and Johann Georg Halske (1814–90); its purpose was to build telegraph installations and other electrical equipment. It soon began spreading telegraph lines across Germany, establishing in 1855 a branch in St. Petersburg for Russian lines and in 1858 a branch in London for English lines, the latter headed by Werner's brother William Siemens (Siemens, Sir William) (1823–83). As the firm grew and introduced mass production, Halske, who was less inclined toward expansion, withdrew (1867), leaving control of the company to the four Siemens brothers and their descendants.

Meanwhile, the company's activities were enlarging to include dynamos, cables, telephones, electric power, electric lighting, and other advances of the later Industrial Revolution. In 1890 it became a limited partnership, with Carl Siemens (Werner's brother) and Arnold and Wilhelm Siemens (Werner's sons) as the senior partners; in 1897 it became a limited-liability company, Siemens & Halske AG.

In 1903 Siemens & Halske transferred its power-engineering activities to a new company, Siemens-Schuckertwerke (having absorbed a Nürnberg firm, Schuckert & Co.); from 1919 on, the two companies were usually chaired by the same officer, always a member of the Siemens family. In 1932, after seven years of collaboration, an Erlander firm, Reiniger Gebbert & Schall, merged with the Siemens interests to form Siemens-Reiniger-Werke AG, engaged in producing medical diagnostic and therapeutic equipment, especially X-ray machines and electron microscopes (electron microscope).

The House of Siemens, as the companies were collectively called, expanded greatly during the Third Reich (1933–45). All plants ran at full capacity during World War II and were dispersed throughout the country to avoid air strikes in 1943–44. At the war's end, Hermann von Siemens (1885–1986), the head of the group, was briefly interned (1946–48), and Siemens officials were charged with recruiting and employing slave labour from captive nations and associating in the construction and operation of the extermination camp at Auschwitz and the concentration camp at Buchenwald. As much as 90 percent of the companies' plants and equipment in the Soviet-occupied zone of Germany were expropriated. The Western powers also removed and destroyed some facilities until the Cold War sparked Western interest in West Germany's economic reconstruction and cooperation. During the 1950s, from its base in West Germany, the House of Siemens gradually expanded its share of the electrical market in Europe and overseas so that by the 1960s it was again one of the world's largest electrical (electrical and electronics engineering) companies.

In 1966 all constituent companies were merged into the newly created Siemens AG. During the early 21st century its products ranged from diagnostic imaging systems, mobile telephones, and hearing aids (hearing aid) to mass transit systems, ground movement radar for airfields, and power generating equipment. The company also designed, built, and operated telecommunications networks (telecommunications network).