This seminar focuses the development of a suitable theory for circuits. Although many models are based in circuits, they are usually seen as graphs, in a somewhat very informal manner. We believe that this can be the cause of many problems and that a suitable formalization is needed. Hence, we present a theory for continuous and discrete time circuits, and prove several results. We also restrict our attention to some special cases and apply this theory to prove basic results concerning Shannon’s General Purpose Analog Computer.