© 1994 SPIE. All rights reserved. Step-scan FT-IR is one of the most versatile and conceptually simple means of applying the power of interferometry to the study of time dependent phenomena. The fundamental advantage of the step-scan method is the separation of the spectral multiplexing from the time domain by collecting the data while the retardation is held constant. This permits impulse-response (time domain, or time-resolved) studies with temporal resolution limited only by the signal strength, detector rise time and speed of the electronics, as well as modulation-demodulation (frequency domain, or phase-resolved) studies at essentially any modulation frequency or by use of multiple simultaneous modulation frequencies, unhindered by the Fourier frequencies of the continuous-scan FT-IR method. In this paper a review of the approaches to the step-scan method is coupled with a report of recent results in the areas of photoacoustic spectral depth profiling and in time-resolved and phase-resolved investigation of the millisecond-tonanosecond dynamics of liquid crystal and polymer films, as well as of transition metal complexes and heme proteins.