Instrumentation and techniques have changed considerably, however, in the intervening twenty years, engendered by the revolution in distributed computing heralded by the advent of the personal computer. In the Fall of 1973, for example, I ran BASIC programs from one of many teletypes connected to a single PDP-8 computer with 8 Bytes of memory. I recently came across loan papers from 1984 for my personal computer (256 Bytes RAM, no hard drive). Analyses which required graphical solution before computers or very astute programming in the first digital decades now can be done using spreadsheets packaged with word processing software; the earlier approaches are no longer of interest, except perhaps to historians.

With changes in instruments came changes in the types of x-ray diffraction experiments a materials scientist/engineer might be required to perform (or at least to interpret). Film-based techniques for powder diffraction fell into relative obscurity while diffractometry (parafocusing geometry) with electronic detectors assumed the principle role. The Second Edition, however, followed the First Edition in treating x-ray diffraction first from the standpoint of photographic methods and second from that of diffractometry; this edition changes emphasis to diffractometry as the primary technique. Practical considerations dictate this change: any x-ray diffraction data collection/analysis that the students subsequently encounter will almost surely be diffractometry, and most instructors emphasize these experiments. If the students are to understand their laboratory sessions, which must begin early in the term, diffractometry must be pushed forward in the course. Unfortunately, this requires a progression different from the very satisfactory approach used before: a single crystal diffracting many wavelengths in all directions, a polycrystal sample diffracting one wavelength in many directions, an apparatus sampling a small section of diffraction cones along a particular direction. I predict, however, that the second decade of the new millennium will see the area detector methods regain the prominence once held by photographic methods.

Yet another challenge was integrating reciprocal lattice treatment of diffraction into the text and adding reforming the material on diffraction from nearly perfect crystals. Since the Second Edition appeared, several topics have grown in importance in different segments of the materials community, diffraction from polymers and small angle scattering, and this is reflected in two new chapters. Finally, materials engineers n6ed some familiarity with transmission electron microscopy, and a couple of weeks, building on reciprocal lattice concepts developed earlier, at the end of a semester course on x-ray diffraction and crystal structure may be all that can be spared. To support this possibility, a brief summary of the high points of TEM in materials characterization is provided.

This revision took a long time to be completed, and I am grateful to the various editors who helped it along: Dan Jorananstad, Rob Merino, Laura Curless and especially Michael Slaughter and Scott Disanno, and also their staffs. I am grateful to the various reviewers for their advice: Z.L. Wang, Steve Spooner and Ray Young. I apologize to those who were forced to wait for me to finish this revision, but the revision was done without taking leave from my academic duties and this delay was the result.

Virtually all scholars blend research and teaching to various degrees, and I am no exception. It is difficult to imagine how this revision it might have been completed without the confidence of having a stable base of research funding provided by the U.S. Office of Naval Research; I am, therefore, very grateful to ONR and George Yoder.

I am very grateful to various mentors who laid the foundation on which this revision is based: Neva Gribble, John Hilliard, M. Meshii, Morrie Fine, Howard Birnbaum, Haydn Chen, Keith Bowen, Jerry Cohen, Brett Boston and Jane Deakins, and R. A. Young. I learned much from many others, and I apologize to them for not explicitly recognizing my debt to them. I am also very grateful to the numerous students from the diffraction classes I have taught and to my own research students, undergraduate, Masters and PhD; it would be hard to find a better bunch anywhere. I am grateful to Vinnie Holohan who helped prepare many of the new figures and to Lisa Novak who helped with the more complex equations.