Organic solvent nanofiltration (OSN) membranes require stable polymers with good solubility to facilitate membrane fabrication. The resultant membrane should also exhibit excellent solvent resistance. Therefore, development of stable polymer membranes for OSN applications has seen dramatic progress in the last decade. Various approaches and membrane fabrication techniques have been employed to develop thin film composite (TFC) membranes, integrally skinned asymmetric membranes (ISA), hollow fiber membranes (HF), and carbon molecular sieve membranes. This chapter will discuss the different types of polymers used to develop OSN membranes and their performance. We will also discuss the various preparation methods and characterization techniques.