In the measurement of tissue impedance, one normally places a system of electrodes on a sample, introduces a known current and measures resulting voltages, which then serve to determine the impedance through some algorithm. To carry out the last step successfully, it is important to have a correct representation of the sample under test. The measurement of the impedance of cardiac tissue illustrates these ideas very well, because the view of the structure of this tissue has changed greatly over the years. Analysis of impedance measurements in cardiac tissue using different representation models of the sample under test are described.