Volume 20 of Reviews in Mineralogy attempted to: (1) provide examples illustrating the state-of-the-art in powder diffraction, with emphasis on applications to geological materials; (2) describe how to obtain high-quality powder diffraction data; and (3) show how to extract maximum information from available data. In particular, the nonambient experiments are examples of some of the new and exciting areas of study using powder diffraction, and the interested reader is directed to the rapidly growing number of published papers on these subjects. Powder diffraction has evolved to a point where considerable information can be obtained from ug-sized samples, where detection limits are in the hundreds of ppm range, and where useful data can be obtained in milliseconds to microseconds. We hope that the information in this volume will increase the reader's access to the considerable amount of information contained in typical diffraction data.