Publisher Summary This chapter focuses on X-ray diffraction and protein structure. X-ray diffraction is now more than 30 years old. Enough X-ray studies of proteins have been made to result in an impressive collection of data. Undoubtedly there is much valuable information concealed in these data, but, to date, the analyses of these data have not generally been carried as far as had been hoped. Where attempts have been made to carry the analyses to more advanced stages, controversies have usually arisen as to the correctness of the conclusions. There are serious technical difficulties that arise in the course of the analyses of X-ray diffraction data of complex structures. In all X-ray diffraction experiments on proteins, a well-defined essentially monochromatic parallel beam of X-rays impinges on a specimen, and the scattered X-rays are recorded on photographic film. Several new and rather attractive pictures of what fibrous protein structures look like have been recently proposed but much more important is the fact that new and better experiments are being performed. Specimen preparation is important; proteins are rather delicate structures and it is essential that the chemical or physical procedures used in the isolation of a fibrous protein be chosen so as to produce a minimum of change from the native state.