Quantum spin tunnelling is a ubiquitous phenomena in all areas of physical sciences. It has many applications in technological devices we use daily. This book is a theoretical description of the basis of quantum spin tunnelling. It surpasses many books in this category in that it contains different approaches and techniques for studying quantum spin tunnelling. It is highly recommended to researchers, graduate and undergraduate students in physics. This book also contains useful information profitable to both specialists and nonspecialists in this field. It can also serve as a review or reference book for many theoretical backgrounds in quantum tunnelling.