The main objective of this paper is to provide a review on the engineering concept of the submerged hollow-fiber module and its operation in MBRs. The main design characteristics of the submerged hollow-fiber membrane modules, the filtration behavior of hollow fibers, the MBR system composition, the main design and operation parameters, and the membrane fouling under the subcritical flux condition in MBRs have been discussed in this paper. It is expected that this paper can provide improved understanding on the key design parameters of the submerged hollow fiber membrane modules, the non-uniform filtration behavior of the hollow fibers, the difference between the actual and observed fiber membrane permeability, the typical MBR system design and operation parameters, the filtration cycle TMP behavior, the operation strategies for the peak flow conditions, and the mechanisms of the development of the long-term membrane fouling under subcritical flux conditions. (C) 2011 Elsevier B.V. All rights reserved.