Pions are excellent probes of the structure of the nucleus and are useful in a variety of particle physics experiments of fundamental significance. For the past 15 years, physicists have been producing copious beams of these mesons by directing intense proton beams into targets. Three “meson factories” are now in operation: TRIUMF, in Vancouver, British Columbia; SIN, in Villigen, Switzerland; and LAMPF, in Los Alamos, New Mexico. A fourth is under construction in the Soviet Union, at Troitsk, south of Moscow. This article surveys the scientific achievements that have come out of pion experiments at the meson factories since they came into operation and explains how the availability of pions has caused medium‐energy physics to burgeon.
Pi meson probes have contributed much to our ideas about how neutrons and protons are distributed and move relative to each other in nuclei, and along with other probes, to our understanding of basic symmetries in particle physics.