As the distinct scientific professions arose in the 19th century, the need for professional societies was felt. Spurred by the momentous discoveries of its last decade (x rays, radioactivity, the electron), the American Physical Society was organized in 1899. The initiative was taken by Arthur Gordon Webster; Rowland and Michelson, the most prestigious physicists in the country, accepted positions as president and vice president. The chief and almost exclusive concern of APS was the encouragement of research in pure physics. This single‐minded behavior resulted in the formation of other societies of physicists: the Optical Society of America in 1916, the Acoustical Society of America and the Society of Rheologists in 1929, and the American Association of Physics Teachers in 1930, with talk of associations for applied physics and mathematical physics. That fragmentation coupled with the financial difficulties of the APS led to the formation of the American Institute of Physics in 1931. AIP played a more active role during the war years than did APS, but after the war the Physical Society grew rapidly. Its 50th anniversary celebration, held at Harvard, was characterized by optimism despite the existence of many new problems.