This book intends to give a systematic exposition of the validity of quantum principles in biological systems. There are two types of applications of quantum theory in physical systems -- the 'trivial applications' and 'non-trivial applications'. Since every object in this universe consists of atoms and molecules, they should be described by the laws of quantum theory -- which we call trivial applications. On the other hand, there exist some systems where the observational results cannot be explained by the laws of classical physics and this requires a change of paradigm -- these are known as non-trivial applications. Many authors pointed out such non-trivial applications of quantum theory to explain how some biological systems function. In this book, we review such kinds of results in a systematic manner which clearly indicates the need to change the paradigm to understand these biological systems better.