Research on quantum dots (QDs) as zero-dimensional nanostructures whose size is not more than a few nanometers has accelerated in the last two decades, especially in the field of medicine. These nanostructures have attracted much attention due to their unique features such as broad excitation range, narrow emission, strong fluorescence, and high resistance to photobleaching. In this chapter, besides common QDs such as cadmium (Cd)-containing semiconductor QDs, other QDs including carbon-based QDs, chalcogenide QDs, and black phosphorus QDs will be discussed. In addition to describing the optical characteristics of these nanostructures, the usual synthesis methods, their modification and cytotoxicity will be reviewed. Finally, the application of each category of QDs in cancer cell imaging will prospect in more detail.