This book presents an updated account of the research and development undergoing in the domain of quantum theory. Quantum theory is the primary theoretical approach that aids one to favorably understand the atomic and sub-atomic worlds which are too far from the cognition on the basis of general intuitions or experiences of daily-life. It is a very rational theory in which a balanced system of hypotheses and relevant mathematical methods facilitate exact explanation of the dynamics of quantum systems whose measurements are systematically influenced by objective uncertainties. Quantum theory has enabled us to apply and regulate novel quantum devices and technologies in quantum optics and lasers, quantum electronics, quantum computing and modern area of nanotechnologies.