The chapter presents some theorems that apply to electric circuits and that center primarily around Thevenin's theorem. Thevenin's theorem takes circuit equivalence to its extreme, by representing any Linear time-invariant system circuit, between any two given terminals, by a linear-output voltage source in the case of resistive circuits. Thevenin's theorem is arguably the most important theorem in circuit analysis, both from theoretical and practical viewpoints. It is therefore discussed at length with several examples that highlight some of its aspects. The discussion of Thevenin's equivalent circuit (TEC) is naturally followed by a discussion of its current-source counterpart, namely, Norton's equivalent circuit (NEC). NEC has the added significance that some circuits may have an NEC, but not a TEC, just as the converse is also true. The chapter ends with the substitution theorem, which is a useful theorem that simplifies the analysis of some types of circuits and can be readily proved using Thevenin's theorem.