Quantitative analysis for one-component systems (for example, additives in polymers, dilute solutions) is well understood, and the mathematical methods needed are comparatively simple. In recent years, software for multicomponent analysis has been made available for Fourier transform infrared (FT-IR) instruments, and two methods are commonly employed, termed K-matrix and P-matrix. Although the mathematical techniques involved have been described, their practical application is less well documented, and there are a number of pitfalls for the unwary analyst. This paper describes some of those pitfalls, and methods for avoiding them. Of great concern is the common occurrence of calibration using carefully prepared standards, followed by the analysis of real samples that contain impurities. This can have disastrous consequences no matter how good the calibration is. The range of concentrations that a multicomponent analysis actually spans, in reality, can be far less than the analyst may think at first sight. Some examples of applications of multicomponent infrared analysis are given, including extensions to the use of newer techniques such as diffuse reflectance, photoacoustic spectroscopy and microscopy.