The study of intersubband transitions in both quantum wells and superlattices has rapidly developed to the point where intersubband photodetectors are becoming attractive for a number of applications. This is particularly significant for Si-based heterostructures due to the advantage of monolithic integration with the conventional silicon signal processing electronics, especially for large-area staring FPAs in advanced IR sensor systems. We review experimental observations of hole intersubband transitions in SiGe/Si quantum wells. In addition to intersubband transitions, we also discuss two normal incident absorption processes: intervalence band transition and internal photoemission from 2-D hole gas in the quantum well. Finally, the progress in the application of SiGe/Si multiple quantum well structures for the fabrication of IR detectors operating in both the 2- to 5-μm and 8- to 12-μm ranges is described.