In the first part of the twentieth century it was believed that all matter was made from atoms composed of just three basic particles - positively charged protons and electrically neutral neutrons forming a central nucleus, with negatively charged electrons circulating in orbits around the nucleus.

However, studies on these seemingly "fundamental" particles in the mid part of the twentieth century soon revealed that the protons and neutrons are in themselves composed of even smaller particles - the so called quarks (which are also present in the cosmic rays originating from outside our galaxy). Protons and neutrons are made from different combinations of just two types (or "flavors" of quark (called "up" and "down" quarks) but four other flavors of quark (called "strange", "charm", "bottom" and "top") have also been identified in cosmic rays and other sources, making six in all.

Together with six types of lepton (the electron, the electron-neutrino, the muon, the muon-neutrino, the tau, and the tau-neutrino) and the intermediate vector bosons (special force carrying entities which allow the quarks and leptons to interact) these particles are currently believed to be the fundamental "building blocks" from which all matter in the universe is composed.

The subject of particle physics deals with the detection and measurement of the properties and behavior of these fundamental particles, along with the four forces by which they are known to interact. These four forces are categorized as gravitational, weak nuclear, electromagnetic, and strong nuclear, though the weak nuclear and electromagnetic forces have been shown to be two different aspects of a single underlying force (the so-called "electroweak" force), and it is believed that this will be combined with the strong nuclear force in a "Grand Unified Theory" (or GUT) at some point in the future - followed by the addition of gravity in a "Theory of Everything" (the eventual demonstration that all four forces are different manifestations of one single underlying force is the ultimate aim of particle physics).

To study these basic particles and forces huge particle accelerators (machines capable of accelerating subatomic particles and smashing them together at very high energies) are required. Because of the scale of these particle accelerators (often tens of kilometers long) they are often built in underground tunnels at international laboratories such as CERN near Geneva in Switzerland, Fermilab near Chicago in the USA, DESY in Hamburg in Germany, SLAC in California, and JINR in Russia and KEK in Japan, as well as others throughout the world.

Typical experiments in the subject often involve hundreds or even thousands of physicists, usually working within multinational research teams at the above laboratories. In addition, engineers and technicians from various different fields (including engineering, electronics, computing etc) are also needed to support the experimentalists, who often work round the clock while their experiments collect data. Processing of the vast amounts of information which is collected is a mammoth task which often pushes computing technology to its limits, and particle physics has often been seen as a driving force towards the development of better and faster computers and computing technology.