Abstract: Nanocrystalline materials have attracted increasing attention due to their unique size and superior physicochemical properties, which enable their broad potential applications in the biomedical fields. The definition, category, preparation and modification of nanocrystals, with particular focus on the recent progress of nanocrystals utilized for tumor diagnosis, cancer treatment, and other medicinal applications are summarized. The fate of nanocrystals in vivo and current challenges in clinical translation are discussed. This review aims to provide insights for further development of nanocrystal delivery systems for tumor diagnosis and treatment.