Synthetic membranes are widely used for water purification and wastewater treatment. Development of membranes for water treatment has provided a method to produce potable water with much lower energy than thermal distillation. This book will describe the basic principles that govern transport across both natural and synthetic membranes in water treatment. The mass and fluid transport equations will be discussed and utilized to predict the passive transport in biological membranes and performance of dialysis, reverse/forward osmosis, microfiltration, ultrafiltration, and nanofiltration membranes. Membrane reactors and biorectors will also be examined.