Subatomic particles are particles that are smaller than atoms. Historically, subatomic particles were considered to be electrons, protons, and neutrons. However, the definition of subatomic particles has now been expanded to include elementary particles, which are so small that they do not appear to be made of anything more minute. The physical study of such particles became possible only during the twentieth century with the development of increasingly sophisticated apparatus. Many new particles have been discovered in the last half of the twentieth century.

A number of proposals have been made to organize the particles by their spin, their mass, or their common properties. One system is now commonly known as the Standard Model. This system recognizes two basic types of fundamental particles: quarks and leptons. Other force-carrying particles are called bosons. Photons, gluons, and weakons are bosons. Leptons include electrons, muons, taus, and three kinds of neutrinos. Quarks never occur alone in nature. They always combine to form particles called hadrons. According to the Standard Model, all other subatomic particles consist of some combination of quarks and their antiparticles. A proton consists of three quarks.