Desalination of seawater using Reverse Osmosis (RO) technology is an important option available to water-scarce coastal regions. Recent full-scale experiences have shown that pretreatment is the key for this application of RO technology. This paper presents the results from a unique series of bench and pilot scale tests conducted in Southern California designed to determine the effect of pretreatment on Seawater Reverse Osmosis (SWRO) performance. The two most commonly used pretreatment schemes for SWRO - conventional pretreatment (coagulation/filtration) and membrane filtration were evaluated. Bench scale tests were conducted for optimization of coagulant dose for conventional pretreatment. A bench scale setup based on a modification of the Batch Internal Recycle Membrane Test (BAIReMT) developed by DiGiano et al (2000) was designed for screening RO membranes. Additionally, size fractionation of the seawater was conducted and RO fouling experiments was performed using the above setup to determine the effect of various size fractions on RO fouling. These size fractions can be related to different levels of pretreatment commonly utilized for seawater RO. The pilot testing is being conducted in two phases: Phase I testing is pretreatment evaluation and Phase II is RO evaluation. During Phase I two filter configurations and Microfiltration were evaluated as pretreatment strategies. In the second phase of pilot testing two RO systems are being operated simultaneously to evaluate the effect of the pretreatments on RO fouling under different operating conditions and water sources. Includes 10 references, tables, figures.