In this penultimate chapter, we shall discuss dualities in Yang–Mills theories with extended supersymmetry in four-dimensional Minkowski space-time. We briefly review supersymmetry multiplets of states and fields and the construction of supersymmetric Lagrangian theories with N = 1, 2, and 4 Poincaré supersymmetries. We then discuss the SL(2,Z) Montonen–Olive duality properties of the maximally supersymmetric N = 4 theory and the low-energy effective Lagrangians for N = 2 theories via the Seiberg–Witten solution. We shall close this chapter with a discussion of dualities of N = 2 superconformal gauge theories, which possess interesting spaces of marginal gauge couplings. In some cases, these spaces of couplings can be identified with the moduli spaces for Riemann surfaces of various genera.