This chapter presents another methodology for the study of transient phenomena caused by the opening or closing of one or more switches, in circuits that are or are not operating in a steady-state, before switching. It is a technique applicable, preferably, to linear circuits and invariant over time, and whose principle is to simulate the closing of a switch by injecting a voltage source into the circuit; or the simulation of opening a switch by injecting a current source. In essentially linear circuits, the procedure is based on the substitution theorem, also called the compensation theorem, and on the superposition theorem. In the development of the method it is assumed that the independent sources present in the circuit are sinusoidal, making it clear, at first, that it applies equally to other types of independent sources. In contrast to the preceding chapters, the various examples presented here are solved using both the classic method of ordinary differential equations and the operational method of Laplace transforms.