Mixed ionic-electronic conductors (MIECs) are examined first with respect to their possible applications and then with respect to their properties. The emphasis is on electrochemical cells in which a MIEC serves either as an electrode or it replaces the solid electrolyte (SE). The dependence of the I-V relations on point defects nature, concentrations and local neutrality is discussed. The question whether it is possible to introduce significant changes in the defect concentrations by doping is examined and shown to have, in many cases, a negative answer. In MIECs the partial electronic (electron/hole) and ionic currents have to be treated separately and so also the corresponding partial conductivities. Methods for determining the partial conductivities are discussed.