Carbon dots (CDs) have been used as a fluorescence probe to sense heavy metal ions. They can be synthesized using cheap sources and offer good optical properties. They provide good photoluminescence properties. The fluorescence emission of CDs can be tuned by controlling particle size, selecting suitable excitation wavelength, and changing the chemical composition via doping with heteroatoms. The synthesis procedure can also affect the optical properties of the synthesized CDs. The unique optical emission enables the sensing of heavy metals, including biological heavy metals (e.g., Fe³⁺, Zn²⁺, Cu²⁺) and toxic metals (e.g., Pb²⁺, As³⁺, Ag⁺, ClO⁻). CDs as fluorescence probes enable a low detection limit and a good linear relationship for a wide concentration range. They can be applied for actual samples with promising properties for assembling electronic devices. This book chapter summarizes the applications of CDs as fluorescence probes to detect heavy metal ions.