Providing a self contained step by step explanation, this book will guide the reader with a basic knowledge of quantum mechanics, to a sufficiently comprehensive level as well as to the frontier of contemporary physics. For the last two decades there has been a ceaseless growth of the area where the path integral (PI) method plays an important role: the main reasons are its intuitive aspect and ease of handling. However, this has raised questions elsewhere and in this book fundamental issues are resolved by starting from the canonical operator formalism to lead the reader to a more comprehensive level. Containing the most recent topics such as the lattice fermion problem in quantum field theory as well as the quantum Monte Carlo method in statistical mechanics this book will suit graduate students of quantum physics.