The aim of this thesis project is to investigate the fouling behavior for the emerging forward osmosis membrane technology and for the well-established reverse osmosis process. Fouling is caused by the attachment of compounds onto the membrane. A better understanding of the mechanism of fouling is important because this phenomenon represents a severe hurdle in membrane filtration. A set of experimental tests were run with two lab scale set up filtration plants, using the same conditions in RO and in FO. The effect of the feed water composition and of initial flux was significant on the flux decline due to fouling of organic molecules.