We present a theory of the low-frequency noise gain gn and photocurrent gain gp in quantum well infrared photodetectors (QWIPs). Expressions for gn and gp in terms of QW capture probability pc and number of QWs N are obtained. These expressions are valid for any number of QWs N≥1 and capture probability 0≪pc≤1. The difference of noise gain from photocurrent gain is due to the discrete structure of generation–recombination centers (QWs) in QWIP. The ratio gn/gp ranges from 0.5 (for pc→1) to 1 (for pc→0). QWIP is well described by a conventional photoconductor theory in the case of low capture probability pc→1, which corresponds to practical QWIPs. The assumptions of the model are discussed in detail, and a comparison with previously published results is made. © 1999 American Institute of Physics.