Abstract: The introduction of counter tubes and the related instrument geometries have made it possible to obtain greatly improved x-ray powder patterns. Most of the important factors that must be understood in x-ray diffractometry are described in terms of their effect on the intensity, peak-to-background ratio, resolution and line shape. These factors are the geometry of the x-ray optical system, the x-ray tube focal spot size, the angular apertures of the primary beam in the focusing and axial planes, the 2 : 1 setting, and the receiving slit. The precision is often limited by the specimen preparation rather than the instrument, and the effects of homogeneity, displacement and transparency, crystallite sizes, and preferred orientation are outlined. A new diffractometer arrangement employing a transmission specimen followed by a focusing crystal monochromator is shown to be a useful supplement to the standard reflecting specimen diffractometer for clay mineral studies. The important characteristics of Geiger, proportional and scintillation counters are described in terms of linearity, quantum counting efficiency, pulse amplitude distribution and counting statistics. A bibliography of key literature references is appended.