Improvements achieved by the new FILMTEC products as well as results of field experiences made with FILMTEC seawater and brackish water reverse osmosis (RO) membranes are presented as per their status as of February 2000, along with the analysis of the relationship between the achieved performance and the pretreatment used in the respective plants, The field performances presented correspond to two commercial RO plants: one is a brackish water RO (BWRO) plant using a fouling-resistant membrane, FILMTEC BW30-365 FR, and the other is a seawater RO (SWRO) plant using the new seawater membrane, FILMTEC SW30HR-320, for higher fouling RO feeds (e.g., open intakes). Possible pretreatment steps are analyzed in relation to measured efficiency to limit the fouling incidence, and possible optimization is reviewed as well.