The basic principle in writing of this CSIR-NET-GATE book is to create a guiding text for students- a text which represents the material in a way that encourages the students for the logical thinking about what they have learned theoretically in their bachelors / masters. At present in India, the national level competitive examinations such as CSIR-NET-JRF, UGC-SLET (SET), IIT-JAM, TIFR, IISc, BARC, ONGC and DRDO exams are of great importance to start advance career after basic graduation/ post graduation studies. These above competitive exams test the in-depth and practical knowledge of various basic concepts. Cracking of any above national competitive examination opens several doors of opportunities in the field of industrial/scientific research and may offer excellent job opportunities in the research as well as teaching field. However, the percentage of qualifying students is varied from the 1-10 %. Hence, an attempt has been made by authors to design the book in such a way that students will become more familiar with the type of conceptual questions that are asked in above said competitive exams.