IR spectroscopy is one of the absorption spectroscopies. As found in a UV-vis spectrometer, IR spectrometers were also developed on the same concept of a UV-vis spectrometer involving a dispersive optical element represented by a prism or a diffraction grating. Fortunately, however, in the IR region, great detectors are available responding a low-frequency modulated light, which enables us to employ a totally different measurement principle using an interferometer, i.e., the Fourier transform technique. In fact, this technique is particularly suitable for the mid- and near-IR regions. Because of the calibration-free character and other many benefits of the Fourier transform IR (FT-IR) spectrometer, the dispersion-type IR spectrometers have mostly been excluded, and instead FT-IR is commonly found in a laboratory. In this chapter, however, a dispersion-type spectrometer is described first to explain the principle of absorption spectroscopy, followed by the FT technique.