In this review article, the ultrasonic characterization of materials using guided plate waves and their usage to elucidate mechanical properties of plate-like structures is reviewed. The purpose here is to summarize and explain the large body of theoretical and experimental work in this developing field. It is also to gain a perspective on recent salient contributions and to analyze the current state of knowledge and practice in guided wave ultrasonics. Models of waves in plates are examined, as are the means to generate and detect them. Their application to several problems of current interest in materials characterization is treated in detail. In particular, composite materials and their inspection and characterization have been a major impetus in the development of guided wave methods. Techniques to inspect composites sensitively and reliably for defects and to probe their micromechanical behavior are a major focus of this article. Also considered are the characterization of adhesive bonds, the measurement of stress and texture, and the detection of defects using guided waves. This review article contains 362 references.