 The review covers the development of cellulose nanocrystals, CNCs, extraction methods and their impacts on surfacial properties, as well as recent strategies for fluorescent functionalization of CNCs through chemical modification, physical adsorption, or in situ growth. Fluorescent CNCs have been applied in metal ion sensing, physiological pH detection, cell imaging, and tumor anti-proliferation, and their highly flexible construction, appealing structural slash optical properties, and outstanding performances make them a promising material for biomedical applications. This article was authored by Rui Li, Yuchen Liu, Farzad Sadie, and others.