Ultrafiltration is a widespread solution for seawater pre-treatment, especially for RO desalination plants and, in particular, for such cases where conventional pre-treatment by flocculation, settling basins, sand filtration, etc. frequently failed to provide a sufficient water quality to the RO membranes. At present there are various types of UF membranes in the market (spiral wound, submerged hollow fine fibre, pressurized hollow fine fibre and multi bore hollow fine fibre membranes). Some of these membranes could not fulfil the expectations with regard to seawater quality and operation reliability which led to the development of the so called multi-bore hollow fine fibre membranes. These membranes have now been in the market for more than three years. The results of pilot test plants as well as of commercially operating membranes show great advantages compared to the conventional pretreatment and the other membranes.