As a general rule, theories uniting special relativity and quantum theory predict the existence of a species of anti-particle for every species of particle. For instance, if such a theory contains electrons, then it also contains their anti-particles, called positrons, for protons, there are anti-protons, and so on.

It is a universal feature of anti-particles that they have the same mass as corresponding particles, and equal, but opposite charges; for examples, electrons and positrons have the same mass, but the electrons carry negative electric charge, whereas positrons carry the exact same amount of positive electric charge. For particles that carry no charges of any kind, particles and anti-particles are identical.