Gravity-driven membrane (GDM) filtration is a membrane process that utilizes hydrostatic pressure to drive the separation. In a long-term operation a permeable biofilm layer is formed on the membrane surface which induces a stable flux that enables longer operation without any cleaning or flushing. The stabilization of the permeate flux depends on the structure and composition of the developed biofilm, which is affected by several including the quality of the feedwater and the operational conditions. In addition to the flux stabilization, GDM process also benefits better organic removal and rejection by the biofilm. The GDM system is very attractive for decentralized water and wastewater treatment, particularly due to the low energy demand and easy maintenance. This chapter overviews the GDM process and discusses factors affecting its performance. © 2021 Elsevier Inc. All rights reserved.