This study investigates gypsum scaling and cleaning behavior in forward osmosis (FO). The results show that gypsum scaling in FO is almost fully reversible, with more than 96% recovery of permeate water flux following a water rinse without addition of chemical cleaning reagents. Parallel comparisons of fouling and cleaning were made between FO (without hydraulic pressure) and RO (under high hydraulic pressure) modes. The shape of the water flux decline curves during gypsum scaling is similar in the two modes, but the flux recovery in FO mode is higher than that in RO mode by about 10%. This behavior suggests that operating in FO mode may reduce the need for chemical cleaning. The role of membrane materials in controlling gypsum scaling and cleaning was investigated using cellulose acetate (CA) and polyamide (PA) membranes. Gypsum scaling of PA membranes causes more severe flux decline and is harder to clean than that of CA membranes. AFM force measurements were performed between a gypsum particle probe and the membrane surfaces to elucidate gypsum scaling mechanisms. Analysis of adhesion force data indicates that gypsum scaling of the PA membrane is dominated by heterogeneous/surface crystallization, while gypsum scaling of the CA membrane is dominated by bulk crystallization and subsequent particle deposition.This finding implies that membrane surface modification and new material development can be an effective strategy to mitigate membrane scaling.