Among different types of nanomaterials, cellulose nanocrystals (CNCs) have attained great attention for their attractive and excellent properties, such as high surface area, colloidal stability, low toxicity, and high mechanical strength. These nanocrystals are obtained from different cellulose sources: wood pulp, microcrystalline cellulose, straw, bacterial cellulose, algae, and plant fibers. CNCs are generally isolated by a top-down approach and can be functionalized to develop advanced materials with improved properties for potential applications in various fields. This chapter elucidates the synthesis approaches (physical, chemical, and enzymatic) and discusses the functionalization techniques (via synthesis, adsorption, and chemical modification). CNCs possess unique and outstanding physicochemical and biological properties. This chapter also discusses the unique and relevant functional (mechanical, thermal, rheological, optical, and biological) properties (cytotoxicity, biocompatibility, biodegradability) of CNCs for various applications. The scope of CNCs as biosensors, scaffolds, liquid crystals, and smart devices and in antimicrobial and food packaging applications is discussed in detail. © Springer Nature Switzerland AG 2022.