The procedure for the analysis of impedance spectroscopy results is outlined. On this basis, the measurement modelling of electrochemical systems is discussed. Attention is focused on several types of ambiguities resulting from the application of electrical equivalent circuits as models, and on the limitations of this type of model. Replacement of equivalent circuit models by more general mathematical models is proposed. This eliminates both the need for detailed assumptions in the measurement modelling and the ambiguity of the results. In this approach, the use of equivalent circuits is reduced to a rough shorthand notation of electrochemical systems. The demarcation between measurement models and process models is also discussed.