This three-lecture series provides a conceptual introduction to the physics of disordered superconductors, emphasizing general principles and their limitations. We begin by reviewing optical sum rules and gauge invariance in metals and superconductors. In addition we discuss the Anderson theorem and the remarkable robustness of conventional s-wave superconductivity against nonmagnetic disorder. Building on this foundation, we then discuss mechanisms of pair breaking, focusing on the effects of magnetic impurities and strong disorder, and introduce the Abrikosov–Gor’kov framework for the suppression of superconductivity. Finally, we turn to unconventional superconductors, where disorder plays a qualitatively different role due to symmetry, topology, and strong correlations. We examine how disorder probes pairing symmetry, modifies low-energy excitations, and can both destroy and induce novel superconducting phenomena. Throughout the lectures, emphasis is placed on unifying principles, experimentally relevant observables, and open questions at the interface of disorder, interactions, and superconductivity.