In this chapter, several analytical techniques to make the best use of the ‘ordinate scale’ of absorbance spectra are described. The techniques are on a concept of the multivariate analysis, which is usually called chemometrics. To employ chemometrics for spectral data analysis, the accuracy of the abscissa axis must be highly rigid. Since the wavenumber accuracy of FTIR spectra is ideally high, which is guaranteed by the calibration using a He–Ne laser, chemometrics spans a huge application field on FTIR spectra. Chemometrics, in fact, enables us to salvage very minute chemical information hidden in an apparent spectrum with ignorable damage on the original data; otherwise important chemical information would be discarded. Highly effective noise reduction of a collection of many spectra is of another great benefit of using chemometrics. On a poor understanding of chemometrics, nevertheless, the chemical discussion would go to a wrong direction. A logically rigid understanding of chemometrics definitely makes spectroscopy more useful in chemistry.