The chemistry of nanocrystals enables the receipt of semiconductor nanoparticles with tunable optical properties. So far most scientific efforts have been focused on wide band gap materials to achieve a bright luminescence and a higher solar power conversion efficiency. Their properties in the infrared range of wavelengths are interesting as well. Two strategies can be used to achieve mid-infrared (mid-IR) transition, either interband transition in narrow band gap material or intraband transition in doped material. In this review, we discuss recent progress to achieve stable doped nanocrystals. We focus on mercury chalcogenide compounds since they are so far the only materials that combine mid-IR absorption with photoconductive properties in this range of energies. We discuss the origin of the doping and its tunability as well as how the doping impacts the optical, transport, and photodetection properties. Finally, we discuss Hg-free alternative materials, and present mid-IR transitions.