A basic concept on the area-specific impedance (ASI) of a single lithium insertion electrode is described in order to characterize lithium insertion electrodes. A common electrode selected to measure the ASI is the zero-strain lithium insertion material of lithium titanium oxide (LTO). The ASI of a single LTO electrode in Omega cm(2) is measured in the state of charge (SOC) ranging from 0 to 100% by using the zero-volt lithium-ion cell of LTO with a symmetric parallel-plate electrode configuration. Because the ASI of an LTO electrode is empirically known to be independent of SOC ranging from 10 to 90% with a margin of 5%, the impedance of a cell having any combination with the LTO electrode can be measured at any SOC of a target electrode. Consequently, the ASI of every lithium insertion electrode can be obtained by the impedance measurements. Significance of determining the ASI of a single lithium insertion electrode is described in terms of polarization and power dissipation generating heat in a full cell, and the methods to characterize lithium insertion electrodes in a basic research are discussed. (C) 2014 The Electrochemical Society.