This book has evolved from lectures devoted to applications of the Wentzel - Kramers - Brillouin- (WKB or quasi-classical) approximation and of the method of 1/N expansion for solving various problems in atomic and nuclear physics. The intent of this book is to help students and investigators in this field to extend their knowledge of these important calculation methods in quantum mechanics. Much material is contained herein that is not to be found elsewhere. WKB approximation, while constituting a fundamental area in atomic physics, has not been the focus of many books. A novel method has...