The book has been titled as mud motors even though there are different types of mud motor and nowadays it has become a common practice to refer PDM as the default mud motor and is the reason PDM is given in the parenthesis to avoid confusion. In this book, mud motor and PDM are interchangeably used. The primary objectives of this book are three fold: 1. To develop a working knowledge of the mud motor 2. To develop procedures and guidelines so that mud motor can be used efficiently within the performance envelop and 3. To develop a clear understanding of the operating principle so that the drilling system can be optimized. Illustrative examples are provided as and when appropriate. Throughout the book we have maintained the engineering perspective related to well engineering aspects.