Kastler, Alfred (1902-1984) was a French physicist who won the 1966 Nobel Prize in physics for his discovery and development of optical methods for studying Hertzian resonances within atoms. His discoveries and new methodologies for measuring weak magnetic fields were the basis for the subsequent development of masers and lasers.

Kastler was born in the village of Gebweiler, Alsace, at the time a part of Germany. After World War I (1914–1918), Alsace reverted to France. The high school he was attending in nearby Colmar was renamed Lycee Bartholdi; there Kastler first became interested in science and math. After graduation, not fluent in French, he determined to become a carpenter but abandoned that idea when he was accepted to the prestigious Ecole Normale Supérieure in Paris. He received his Ph.D. degree in physics from there in 1936.

Kastler taught in various universities until 1941, when he returned to Paris and became director of the Hertzian spectroscopy group, as well as assistant professor of physics at the École Normale Supérieure. He became full professor in 1945 and remained there until 1968, after which he retired and became director of research at the National Center for Scientific Research.

Kastler's most outstanding work involved research on electron energy levels. In particular, he developed two techniques for exciting atoms so that they would emit energy while moving in very exact ways. The first method, double resonance, uses a light beam and a radio frequency field to investigate atoms as they move to a higher energy level and then back to a state of lower energy. Using the double resonance method, Kastler developed his second method, optical pumping by shining polarized light on a group of atoms. Kastler's discoveries led to several practical inventions, including the atomic clock, and masers and lasers, which use the energy emitted by excited atoms.

In 1966, Kastler became the first French citizen to be awarded a Nobel Prize in 37 years.