Gustaf Ising (or Gustav Ising in some publications), born on 19 February 1883 in Finja, died 5 February 1960 in Danderyd, was a Swedish accelerator physicist and geophysicist. Ising earned his first academic degree (filosofie kandidat/Bachelor of Arts) at Uppsala University in 1903 and continued studying at Stockholm University receiving his Ph.D. in 1919, later becoming a full professor in 1934. He is best known for the invention of the linear accelerator concept in 1924, which is the progenitor of all modern accelerators based on oscillating electromagnetic fields. His article was then taken up and turned into practice by Rolf Widerøe, also starting the development of cyclic accelerator structures like the cyclotron and the betatron. He was elected to the Swedish Academy of Sciences in 1935, being a member of the Nobel Committee for Physics from 1947–1953, together with former Nobel Laureate and chairman Manne Siegbahn, Svante Arrhenius, Erik Hulthen, Axel Edvin Lindh, Ivar Waller, and Carl Borelius.