This article presents a review of the recent development of mixed conducting ceramic materials and membranes, and their potential applications in separations and chemical reactions. A basic knowledge of the materials, including their structure, composition, and properties, is first introduced briefly, followed by a short discussion on the relation between the material structures and properties as well as strategies for new material development and screening. The main part of this review gives an intensive discussion on the progress achieved up to early 2005. Performances of the mixed conducting materials for both oxygen and hydrogen and their function as membranes or membrane reactors are summarized. Some examples in gas/vapor separations and catalytic chemical reactions are presented. Current problems related to the fabrication of the mixed conducting membranes are addressed and future technology challenges are also outlined.