An important application of FTIR is quantitative spectroscopy, where infrared spectra are used to determine the concentrations of chemical species in samples. Entire books have been written on this subject [1], and a lengthy description of quantitative spectroscopy is beyond the scope of this volume. My goal here is to introduce you to the fundamentals of quantitative spectroscopy by focusing on the determination of one chemical species at a time. Quantitating multiple chemical species at a time via infrared spectroscopy is doable given proper technique, equipment, software, and skill and is discussed elsewhere [1].