In Chapter 1 solid electrolytes are reviewed. The historical development, mechanism of ionic conduction in solids and the properties and applications of important solid electrolytes are given. Chapter 2 discusses impedance methods as applied to electrochemistry and the equivalent circuit representation of the solid electrolyte/electrode interface. The application of a computer program that gives theoretical impedance and/or admittance plots is also described. The application of four-electrode conductivity measurements to solid electrolytes is described in Chapter 3. Since this is the first time these four-electrode measurements have been employed with solid electrolytes, a detailed review of the theoretical calculations and the errors involved are presented. The results from the experiments on the solid electrolyte PyAg₅I₆(Py=C₅H₆N) are also given. In Chapter 4, the cathodic deposition of silver from PyAg₅I₆ and its subsequent recovery by anodic polarisation are examined. The behaviour of solid silver and silver painted electrodes in this perspective is compared. The solid electrolyte galvanic cell AgmidPyAg₅I_6midMe₄NI₅, C is investigated in Chapter 5. Various cell parameters are measured and the charge/discharge characteristics examined. Finally in Chapter 6 the behaviour of zeolites as solid ionic conductors is reviewed and the results of some preliminary studies on zeolite-Y are presented.