Abstract: Chitooligosaccharides (COSs) are homo- or heterooligomers of N-acetylglucosamine and D-glucosamine obtained via degradation of chitin or chitosan. COSs are useful for remarkably wide spectrum of applications in the pharmaceutical, food, and agricultural industries due in part to their antimicrobial, antioxidant, antitumor, and immunomodulatory activities. Enzymatic, physical, and chemical methods for the preparation of COSs have been reported. At present, the preparation of COSs by any single method faces limitations, including difficulty in obtaining target products with specific degrees of polymerization and acetylation. Therefore, COSs preparation protocols have shifted from single method strategies to explorations of cooperative catalytic systems. COSs purity can also be improved by incorporating separation and purification techniques including ultrafiltration, activated carbon adsorption, and chromatography. In this paper, progress in COSs preparation and purification methods and COSs applications are reviewed, with the objective of providing a theoretical basis for improved production and expansion of applications for high quality COSs.