The x-ray fluorescence spectrometer consists of three main parts: the excitation source, the specimen presentation apparatus, and the x-ray spectrometer. The function of the excitation source is to excite the characteristic x-rays in the specimen via the x-ray fluorescence process. The specimen presentation apparatus holds the specimen in a precisely defined position during analysis and provides for introduction and removal of the specimen from the excitation position. The x-ray spectrometer is responsible for separating and counting the x-rays of various wavelengths or energies emitted by the specimen. In this book the term x-ray spectrometer denotes the collection of components used to disperse, detect, count, and display the spectrum of x-ray photons emitted by the specimen. When referring to the entire instrument, including excitation source, sample presentation apparatus, and x-ray spectrometer, the term x-ray fluorescence spectrometer will be used. In this latter sense the term x-ray fluorescence analyzer is sometimes encountered in the literature.