Luis Walter Alvarez (June 13, 1911 – September 1, 1988) was an American experimental physicist, inventor, and professor who was awarded the Nobel Prize in Physics in 1968 for his discovery of resonance states in particle physics using the hydrogen bubble chamber. In 2007 the American Journal of Physics commented, "Luis Alvarez was one of the most brilliant and productive experimental physicists of the twentieth century."

After receiving his PhD from the University of Chicago in 1936, Alvarez went to work for Ernest Lawrence at the Radiation Laboratory at the University of California, Berkeley. Alvarez devised a set of experiments to observe K-electron capture in radioactive nuclei, predicted by the beta decay theory but never before observed. He produced tritium using the cyclotron and measured its lifetime. In collaboration with Felix Bloch, he measured the magnetic moment of the neutron.

In 1940, Alvarez joined the MIT Radiation Laboratory, where he contributed to a number of World War II radar projects, from early improvements to Identification friend or foe (IFF) radar beacons, now called transponders, to a system known as VIXEN for preventing enemy submarines from realizing that they had been found by the new airborne microwave radars. The radar system for which Alvarez is best known and which has played a major role in aviation, most particularly in the post war Berlin airlift, was Ground Controlled Approach (GCA). Alvarez spent a few months at the University of Chicago working on nuclear reactors for Enrico Fermi before coming to Los Alamos to work for Robert Oppenheimer on the Manhattan Project. Alvarez worked on the design of explosive lenses, and the development of exploding-bridgewire detonators. As a member of Project Alberta, he observed the Trinity nuclear test from a B-29 Superfortress, and later the bombing of Hiroshima from the B-29 The Great Artiste.

After the war Alvarez was involved in the design of a liquid hydrogen bubble chamber that allowed his team to take millions of photographs of particle interactions, develop complex computer systems to measure and analyze these interactions, and discover entire families of new particles and resonance states. This work resulted in his being awarded the Nobel Prize in 1968. He was involved in a project to x-ray the Egyptian pyramids to search for unknown chambers. With his son, geologist Walter Alvarez, he developed the Alvarez hypothesis which proposes that the extinction event that wiped out the non-avian dinosaurs was the result of an asteroid impact.

Luis Walter Alvarez was born into a Catholic family in San Francisco on June 13, 1911, the second child and oldest son of Walter C. Alvarez, a physician, and his wife Harriet (née Smyth). His grandfather, Luis F. Álvarez, was a physician born in Asturias, Spain, who lived briefly in Cuba, settled in the United States, and discovered a better method for diagnosing macular leprosy. He had an older sister, Gladys, a younger brother, Bob, and a younger sister, Bernice. His aunt, Mabel Alvarez, was a California artist specializing in oil painting.

He attended Madison School in San Francisco from 1918 to 1924, and then San Francisco Polytechnic High School. In 1926, his father became a researcher at the Mayo Clinic, and the family moved to Rochester, Minnesota, where Alvarez attended Rochester High School. He had always expected to attend the University of California, Berkeley, but at the urging of his teachers at Rochester, he instead went to the University of Chicago, where he received his bachelor's degree in 1932, his master's degree in 1934, and his PhD in 1936. As an undergraduate, he belonged to the Phi Gamma Delta fraternity. As a postgraduate he moved to Gamma Alpha.

In 1932, as a graduate student at Chicago, he discovered the field of physics and had the rare opportunity to use the equipment of legendary physicist Albert A. Michelson. Alvarez also constructed an apparatus of Geiger counter tubes arranged as a cosmic ray telescope, and under the aegis of his faculty advisor Arthur Compton, conducted an experiment in Mexico City to measure the so-called East–West effect of cosmic rays. Observing more incoming radiation from the west, Alvarez concluded that primary cosmic rays were positively charged. Compton submitted the resulting paper to the Physical Review, with Alvarez's name at the top.

Alvarez was an agnostic even though his father had been a deacon in a Congregational church.