Energy harvesting is now an established field, and the linear equivalent circuit models for single-and multiple-degree-of-freedom systems are being routinely used to assess the energy harvesting efficiency of various interface and electronic circuits. However, as the field of energy harvesting moves toward more complex systems such as nonlinear energy harvesting and fluidic energy harvesting, modified equivalent circuits are required to effectively capture the behavior of these devices. This article presents two general methods (a system-level approach and a dependent source equivalent approach) that can adequately model the behavior of more advanced harvesters. These approaches can also be easily used to model single-and multiple-degree-of-freedom linear harvesters. Equivalent circuits for both piezoelectric and electromagnetic harvesters are discussed. Four case studies are presented to illustrate the application of these equivalent circuit approaches including the following: (1) a piezoelectric Duffing harvester, (2) an electromagnetic Duffing harvester, (3) a simplified aeroelastic harvester, and (4) a piezoelectric Duffing harvester with a rectifier circuit.