Water treatment in membrane plants implies pre-treatment and often post-treatment also. Pre-treatment is mandatory because membrane systems are susceptible to fouling. The extent of physical and chemical pre-treatment depends on the quality of raw water, e.g. well water typically require minimum pre-treatment, whereas wastewaters require extensive pre-treatment. Both physical and chemical pre-treatment processes and technologies are discussed in detail. Post-treatment technologies are also detailed. Post-treatment is required depending on the application of product water at the point-of-use, e.g. RO permeate is remineralised prior to distribution for potable use. Conversely, RO product water is polished to further demineralise it for applications that require high-purity water (pharmaceuticals) or ultrapure water (microelectronics). Membrane scaling/fouling, process design guidelines to minimise fouling and membrane cleaning are discussed in detail. This chapter describes in detail membrane system designs including important design features and system controls.