This book has three chapters. At the beginning of first chapter introduction of quantum dots, quantum size effects have given. CdS/ZnS are the core/shell NCs hence in latter section core/shell NCs, types of core/shell NCs explained briefly. At the end of chapter literature survey on CdS/ZnS core/shell NCs has given. The structural changes in CdS with ZnS cladding studied by recording x-ray diffraction, transmission electron microscopy analysis which also confirms shape and size of nanocrystals hence those techniques explained in second chapter. Changes in optical and electronic properties studied by optical absorption, photoluminescence and time resolved spectroscopy those explained at the end of second chapter. In the third chapter the results of work are discussed. In this book method of synthesizing CdS nanocrystals (NCs) with size 1.9 nm and CdS/ZnS core shell NCs are discussed in order to examine the effect of lattice strain due to lattice mismatch of 7%.