The present book grew out of a lecture course given at the Institute for Advanced Studies in Basic Sciences, Zanjan, Iran in the summer of 1999. The intent at the outset was to present some of the basic results and methods of quantum theory of tunneling without concentrating on any particular application. It was difficult to decide what topics should be treated at length and which ones should be omitted from the discussion. Thus my main area of interest, the quantum theory of dissipative tunneling, was left out completely since even an introductory survey of the subject would have nearly doubled the size of the book.