In the previous Chapters 3 and 4, we extensively discussed the electromagnetic and strong interactions, with Chapter 5 explaining that in processes involving quarks and hadrons, both forces must be treated together. Now, in Section 6.1, we turn to the weak interaction, whose laws can also be most clearly formulated in purely leptonic processes. When extending to quarks and hadrons in Section 6.2, Cabibbo’s modification of the Feynman rules for first and second-generation quarks is central. In the following section, this idea is extended to three generations of quarks. The central themes of flavor oscillations and CP violation are discussed in detail. Subsequently, in Section 6.4, we address neutral currents and the unification of electromagnetic and weak interactions in the Glashow, Salam, andWeinberg model (GSW model) and finally discuss in Section 6.5 experimental tests of this theory.