Different from traditional chemical surface modification, localized modification of the reducing end groups of cellulose nanocrystals (CNCs), i.e. the active aldehyde groups, provides new opportunities for diverse functional applications of this renewable nanomaterial without altering its surface chemistry and properties. Numerous reviews have deeply discussed the surface modification of the hydroxyl groups of CNCs, but no critical comment has been reported on the reducing end modification approach. This review is a comprehensive summary on the modification of the CNC reducing end, presenting the reaction mechanisms and conditions, discussing the different chemical modification strategies and characterization techniques, potential applications and future challenges in this field. In addition, the comparison between surface and end modification strategies of CNCs will highlight the potential of reducing end-functionalized CNCs to be used in various applications as an alternative to traditional surface-modified CNCs, or as additional functional nanoparticles for the design of advanced functional materials.