This chapter introduces the quantum field theory for weak interactions and the concept of electroweak unification, following a review of QED and QCD. It begins with a historical overview and examination of weak decay processes like pion and muon decay, contrasting their long lifetimes with those of strong and electromagnetic decays. The discussion covers early models, including Fermi’s vector-vector interaction, the discovery of parity non-conservation, and the modern vector-axial-vector structure involving chiral projection operators and massive W and Z bosons. The chapter derives the total decay rate for muon decay to determine the Fermi constant and analyzes charged pion decay to introduce the pion decay constant. Furthermore, it explores electron-neutrino and neutrino-quark scattering, demonstrating their use in constraining parton distribution functions. Finally, it introduces weak neutral currents and the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which describes quark mixing and flavor-changing charged currents.