Semiconductor nanocrystals are described as a state of matter that are intermediate between individual molecule and bulk.The small size and high optical activity of certain doped nanoparticles make them interesting for applications in optoelectronics, catalysis and fluorescence microscopy. In nonmaterial extremely high surface to volume ratio is obtained leading to an increase in surface specific active sites for chemical reactions and photon absorption to enhance the reaction and absorption efficiency. Blue-shift in the optical absorption, PL spectrum etc. is affected by the particle size. The incorporation of transition metal ions into nanocrystals induces radiative recombination of excited electron-hole pair, which shifts photoluminescence wavelength and increases quantum efficiency. This book provides information on the XRD,SEM,TEM,FTIR,optical absorption, PL spectra,their mechanism, theories and experimental detail of ZnS nanoparticles doped with Mn. A very cost effective chemical route method to synthesize the nanoparticles is explained.This book is very useful for those research scholars and post graduate students who want to work on optical properties of nanoparticles.