A major challenge facing widespread implementation of membrane-based water purification is fouling, which results in increased operating costs and reduced membrane lifetime. This thesis focuses on various methods, including novel membrane surface modifications and polymers that resist degradation when exposed to oxidizing agents used as disinfectants, to alleviate membrane fouling. Fouling-resistant ultrafiltration membrane coatings were prepared from poly(ethylene glycol) diglycidyl ether-crosslinked chitosan (chi-PEG hybrid). Composite membranes were prepared for oil-water emulsion filtration by coating the most promising chi-PEG hybrid onto a polysulfone ultrafiltration membrane. Optimization of the coating layer thickness led to composite membranes that exhibited water flux values more than 5 times higher than that of uncoated membranes after one day of oily-water crossflow filtration. The organic rejection of the coated membranes was also higher than that of the uncoated polysulfone membranes. Polydopamine (PDOPA) deposition was discovered to reduce fouling in water purification membranes. PDOPA was found to deposit from solution onto virtually any surface. When deposited on water purification membranes, PDOPA rendered the membrane more hydrophilic and less susceptible to fouling. Moreover, covalent binding of other molecules, such as amine-terminated poly (ethylene glycol) (PEG), to PDOPA is simple and performed using benign chemicals and conditions. Commercially-available polymeric membranes were modified with polydopamine, and all showed improved fouling resistance while filtering oil-water emulsions. To demonstrate the versatility and ease of PDOPA modification scalability, PDOPA was deposited on entire membrane modules, and the resulting modified module exhibited improved fouling resistance. Finally, high ion rejection, chlorine-tolerant sulfonated polysulfone thin-film composite membranes were prepared and characterized. Interestingly, freestanding thick sulfonated poly(arylene ether sulfone) (BPS) films exhibit nearly neutral electrostatic charge, even though sulfonation introduces fixed negative charge into the polymer structure. As a result, charge exclusion ion partitioning is not a dominant rejection mechanism in these films. However, composite membranes prepared from a BPS coating layer and a porous Udel polysulfone support exhibit a negatively charged surface and, presumably, charge exclusion would be a more important partitioning mechanism for these membranes. Therefore, thick BPS films do not exhibit certain drawbacks, such as reduced salt rejection of mixed-valence feeds, that are observed in BPS thin-film composite membranes. / text