This project details the decay properties of the Higgs boson within the Standard Model and beyond. Using perturbative quantum field theory and Feynman diagrams, we compute the scattering amplitude and decay width of the Higgs boson decaying into a fermion-antifermion pair and pairs of $W$-and $Z$-bosons, gluons and photons. In particular, we give a detailed calculation of the decay into gluons and photons, which, at leading order, is a one-loop process. Additionally, we examine novel decay modes of the Higgs boson into dark matter particles, introducing potential models of physics beyond the Standard Model. The implications of these decay modes on the Higgs bosons total width and branching ratios are analyzed and we give constraints on dark matter particle paramaters such as coupling and mass. This work studies the Higgs boson and its role as a probe of both current Standard Model physics and extensions beyond it.