Infrared absorption spectroscopy is a rapid, economical and non-destructive physical method universally applicable to structural analysis. The technique is so versatile that it can be used both as a source of the physical parameter of crystal lattice determinations, and as a means of eliciting purely empirical qualitative relationships between specimens. It is an intrinsically simple technique that deserves to be more widely used in clay mineralogy and soil science, and this chapter aims to provide some basic information to help bring this about. It is proposed to indicate briefly the basic principles of IR, describe the instrumentation and methods, present some typical spectra of standard minerals, and finally to show spectra of a few representative soil clays, indicating how they may be used to assess the constituent minerals.