Interferometry, in combination with the Fourier transform, is a powerful tool for IR spectroscopy. However, most commercial MIR spectrometers are not well suited to measuring spectra of dynamic systems. This disadvantage is due to the rapid scan mode of operation, in which the scanning mirror of the interferometer moves at constant velocity, v. As a consequence, each IR wavelength, λ, is modulated with its own Fourier frequency (f = 2v/λ). This time dependent modulation of the IR, caused by the rapid scan, becomes increasingly more difficult to separate from time dependent variations of the IR intensity caused by a dynamic system under study, as the time resolution desired approaches the Fourier modulation periods. A convenient solution to this problem is the use of step-scan interferometry. In this mode of operation, the moving mirror of the interferometer is stopped at evenly spaced intervals of the HeNe reference laser interference pattern while data are collected. As a consequence, the IR light is not modulated by the interferometer, and temporal intensity variations can be related to the dynamic system under study. Applications include photothermal/photoacoustic spectroscopy, phase-resolved spectroscopy, and classic time-resolved spectroscopy (i.e., pulsed excitation).