Heavy metal ions including Cu²⁺, Zn²⁺, Pb²⁺, Hg²⁺ and Ag²⁺ are vital minerals that are required for the proper functioning of various systems in living organisms. However, an excess amount of these ions may result in severe health problems and malfunctioning of different processes. Furthermore, the presence of these heavy metal ions in underground water also poses environmental threats. Ultra-sensitive detection of these heavy metal ions is therefore, quite important. Semiconductor quantum dots (QDs), especially II–VI QDs can play a very important role in the optical detection of these ions. Moreover, the surface of QDs can be engineered with specific molecules for selective and sensitive detection of a particular heavy metal ion in the presence of other ions. Below we discuss the luminescence and fluorescence detection methods of some important heavy metals including copper, mercury and lead in solution and the ion sensing ability of II–VI QDs. Mechanisms of operation of ion sensors based on QDs, features and advantaged of ratiometric ion sensors for detecting metal ions are also considered in this chapter.