The interdisciplinary area of science and engineering dealing with solid electrolytes and mixed conductors is frequently known as solid-state ionics. It concerns materials that show rapid ionic motion with or without electronic conductivity, from basic science through application. Interest in solid-state ionic materials has continued for the past few decades due to several important, promising applications, such as fuel cells, batteries, sensors, and electrochemical pumps. The principle behind these applications is simply either the Nernst law (as exemplified by Equation 1 in the article by Singhal in this issue) or Faraday's laws of electrochemistry (which connect current flow to mass flow), as applied to a cell consisting of an electrolyte and two electrodes. However, the technological issues are complex, and demands on materials can be very diverse, as illustrated by the five articles in this issue. These articles are based, in part, on the invited talks presented at a symposium in April 1999 on the same subject at Lehigh University to commemorate G.C. Farrington's inauguration as its president.