Constant-Phase Elements (CPE) are often used to fit impedance data arising from a broad range of experimental systems. Four. approaches were used to interpret CPE parameters associated with the impedance response of human skin and two metal oxides in terms of characteristic frequencies and film thickness. The values obtained with each approach were compared against independent measurements. The power-law model developed recently by Hirschorn et al.(1,2) provided the most reliable interpretation for systems with a normal distribution of properties. Readers are cautioned that the CPE parameter Q does not provide an accurate value for capacitance, even when the CPE exponent alpha is greater than 0.9. (C) 2013 The Electrochemical Society. All rights reserved.