We use the Shockley-Ramo (SR) theorem to calculate terminal currents in single photon excited APDs. A unified notation for the SR theorem is introduced, and we derive the theorem in those particular ways found useful for displaying its limits of applicability. Furthermore some analytically solvable examples that clarify the implications of the theorem have been studied. Finally, some tests with this method on an APD have been carried out, using an existing Monte Carlo code, and the results are discussed.