Membrane technology has been successfully applied in many different industrial fields and in some of them, such as desalination, it can be considered a dominant technology. Membranes are produced using a wide variety of techniques which mainly depend on the membrane material employed and on the applications. Two different configurations, flat and hollow fibers, can be identified and depend mainly on the type of the process and application. This Special Issue will highlight the importance of the use of hollow fiber membranes and their varied applications. Both original contributions and reviews are welcome related to: (1) preparation of hollow fiber membranes of different materials, such as polymeric, inorganic or mixed matrix membranes; (2) applications using hollow fiber membranes for wastewater treatment, drinking water production, water reuse, removal of toxic metals from water, volatile organic compounds removal from aqueous or gas streams, gas separation, CO2 removal, and production and recovery of high-added value products from wastewater.