This book provides the specialists in the computer networking field with the necessary knowledge to understand and use a network simulation facilities. It begins, in chapter one, by defining the important terminologies and concepts in this field, then exploring the various mathematical methods to build a network simulator. Chapter two, gives an overview of the different simulation tools available nowadays together with their important features. Chapter three presents the details of the adopted simulation package in this book, OPNET IT GURU. In chapter four, the basic simulation strategies of the free academic version OPNET IT GURU is detailed. Finally, different lab experiments and case studies are illustrated to enrich reader experience. This book could be useful in teaching different courses, for both under and postgraduate levels, such as: computer networking, network simulation and computer networking laboratory.