The optical and vibrational properties of small CdSe and CdS particles embedded in a glass matrix and as a colloid have been studied as a function of pressure up to 90 kbar using Raman scattering and photoluminescence techniques. We will discuss the use of high pressure optical spectroscopy techniques, where the sample is contained in a diamond anvil.cell with optical access, to study the nature of the electronic states in semiconductor nanocrystals. Raman scattering is employed to establish the enhanced pressure stability of the wurtzite phase in the II-VI nanocrystalline composites. Photolurninescence is used to study the energies of electronic states. The wurtzite to rocksalt phase transition behavior in the nanocrystallite systems is quite different from that in the bulk material. This different behavior is attributed to a large number of defects (vacancies) in the nanocrystallite system. This work demonstrates that the main defects in the CdS glass composites are cadmium vacancies; while in CdSe Selenium vacancies exist in the conduction band. The pressure dependence of the Huang-Rhys parameter, characteristic for the strength of the electron-phonon coupling, will also be discussed for the nanocrystalline samples versus the bulk material.