The present book is divided in six chapters. Chapter-1 is based on the historical development with reviewed literature of standard model, physics beyond the standard model, quaternions, octonions, monopoles and dyons. Chapter-2 describes the solutions of wave equation and other field equations of monopoles and dyons in terms of physical octonion variables, i.e. the octonion electrodynamics has been analyzed in terms of compact simple and manifestly covariant way. In chapter-3, we have made an attempt to investigate the generalized split octonion electrodynamics for dyons. In chapter-4, we have discussed the octonion electrodynamics in homogeneous (isotropic) and chiral medium. In Chapter-5, we have studied the abelian and non-Abelian gauge theory of dyons with the application of split octonions, and reformulated the theory of strong interaction (i.e. quantum chromodynamics (QCD)). It is shown that in this theory the gluonic field strength tensor of QCD behaves like to the electromagnetic field strength tensor of QED. Chapter-6 describes the role of octonions in physics beyond standard model, i.e. GUTs, supersymmetry, dark matter, superstring theory etc.