Elementary Particle Theory deals with the fundamental building blocks of matter and attempts to explain their properties. These properties are not only important for present-day experiments, but also impact the early Universe—influencing the creation of protons, neutrons, and electrons, as well as determining the amount of dark matter in the universe. Therefore, the study of Elementary Particles leads not only to a deeper understanding of the Universe today, but also the Universe of the past.

Generally, a particle physicist studies and does research in one or a few of the following categories:

General Theorists
General Theorists have an interest in exploring the deeper physical meaning of the theory itself, typically exploiting various mathematical techniques to gain insight into its physical nature. This type of particle physicist is often found producing general theorems or equations that others may use to simplify their calculations.
Model Building
A Model Builder attempts to construct an extension of the Standard Model of Particle Physics in such a way as to either fix a problem inherent in this theory (or one of its extensions), or to explain some phenomenon that the experimentalists have detected.
A Phenomenologist is usually occupied with explaining some experimental phenomenon by doing an actual calculation of an experimental process using a particular model. The end result is an actual number that can be compared with current experimental data or relayed to experimentalists so that they may design an experiment to test the validity of the predicted value.
String Theory
String Theorists attempt to discover the implications of treating the fundamental building blocks of nature as extended objects (having atleast one length dimension), instead of, like other particle physicists, treating them as point particles (objects of zero size). String Theory is still young and consequently not enough is known about it: in fact, it remains a major goal of String Theorists to demonstrate that the theory describes nature.