This book targets a broad readership for young researchers(Postgraduate students) in X-ray crystallography and Quantum Chemistry as it contains a wide and comprehensive coverage of all important aspects of the subject including up-to- date survey of recent revolutionary advances. Molecular structure determination using crystallographic technique of pyrazolo[3,4-d] pyrimidines, naturally occurring drugs as well as synthetic molecular crystals has been discussed.The optimized structures and the electronic properties of the molecules using quantum mechanical methods have also been discussed and results have been compared with crystallographic data, which could enable one to understand and evaluate different quantum mechanical methods. It should be a valuable asset to established researchers in this area due to very detailed explanation of the behavior of aromatic π-π interactions on the basis of experimental and theoretical results. The crystallographic and theoretical investigations on some synthetic and natural drugs attract the research in the area of drug design using Molecular modeling of the reported compounds with suitable targets.