Volume I/21A is the first in a series of handbooks on elementary particles. It provides the present state of theoretical and experimental knowledge in particle physics, just in time with the start-up of LHC which promises to settle some of the present-day issues in understanding elementary particles and the fundamental forces between them. It starts with a general representation of gauge theories and the Standard Model which unifies the strong, weak, and electromagnetic interactions. The standard model of electroweak interactions, and QCD, the theory of the strong interaction, are dicussed in detail. In addition to analytic approaches to nonperturbative QCD, simulations of QCD on a discrete space-time lattice are treated. Results of experimental precision tests for the electroweak standard model as well as of relativistic nucleus-nucleus collisions (including a discussion of the QCD matter phase diagram) follow next. Concepts which go beyond the Standard Model, like supersymmetry, strings, grand unification, etc., are introduced, and the interrelation between particle physics and cosmology is discussed.Further topics are symmetry violations (parity P, charge conjugation C, and combined symmetry PC), the mixing of quark flavours, and experimental results on neutrino masses and oscillations. Finally, an outlook on the future of particle physics is provided.