This book presents and analyzes the influence of small size particles of lead, cadmium and silver sulfide on the properties of nonstoichiometric semiconductors. Important nonstoichiometry aspects in nanostructures are discussed, such as the distribution of sulfur atoms in nanofilms, a non-periodic distribution of the atomic planes in nanoparticles, interdependent changes in crystal structure of nanocrystalline material. Tuning the stoichiometry allows to obtain superionic conductivity and catalytic activity under visible light. The wavelength of the luminescence of nanoparticles changes with the size of the nanoparticles. Various methods to prepare nanostructured sulfides are described. Special attention is given to the hydrochemical bath deposition as a universal method for the synthesis of sulfides as nanofilms, stable colloidal solutions, quantum dots, isolated nanoparticles with a protective shell and heteronanostructures. The effect of nanoparticle size and nonstoichiometry on the band gap, optical and thermal properties of nanostructured sulfides is also considered. The novel applications of sulfide nanoparticles in nanoelectronics, catalysis, nanobiology and nanomedicine are sketched.