Doped nanomaterials represent a class of nanomaterials with many interesting and unique properties that are useful for important technological applications. As an example, Cu-doped ZnS nanocrystals, the bulk analog of which is widely used as a phosphor material, exhibit novel optical properties and are of interest for opto-electronics applications such as electroluminescence devices. In this article, we provide a review of the background and recent research activities on Cu-doped ZnS nanocrystals with emphasis on correlating their structural properties and optical properties. In addition, dynamic properties of charge carries and relevant electronic energy levels are discussed to provide deeper fundamental insights into their electronic and optical properties.