Castaing’s successful implementation and application of the electron probe microanalyzer in 1950 stimulated a flurry of development activity around the world. The later versions of this instrument represented a truly international effort, with significant contributions by scientists from Europe, Asia, and North America. If the probe-forming system of the instrument was its heart, the X-ray wavelength spectrometer was its soul. This article reviews some of the history of spectrometer developments—lthrough the “golden years” of microprobe development, namely the dozen or so years following the publication of Castaing’s thesis, to the present. The basic physics of spectrometer and crystal design is reviewed. Early experimental devices, such as those developed by Castaing, Borovskii, Wittry, Duncumb, and Ogilvie are reported. Examples of commercial spectrometers such as those by ARL, MAC, Microspec, and Peak are described. Recent developments such as the combination of grazing-incidence optics with flat crystal spectrometers are noted, and the properties and uses of doubly curved crystals are discussed. Finally, the continued development of doubly curved crystal configurations, such as the “Wittry geometry” for scanning monochromators, and point-to-point focusing diffractors for producing small monochromatic X-ray probes to provide improved detection limits for microanalysis are considered.