Abstract: Chitooligosaccharides (COS) is homo- or heterooligomers of N-acetylglucosamine and D-glucosamine obtained via degradation of chitin or chitosan. COS is 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 COS has been reported. At present, the preparation of COS by any single method faces limitations, including difficulty in obtaining target products with specific degrees of polymerization and acetylation. Therefore, COS preparation protocols have shifted from single method strategies to explorations of cooperative catalytic systems. COS purity can also be improved by incorporating separation and purification techniques including ultrafiltration, activated carbon adsorption, and chromatography. In this paper, progress in COS preparation and purification methods and COS applications are reviewed, with the objective of providing a theoretical basis for improved production and expansion of applications for high quality COS.