This text presents the basic concepts of discrete event simulation using ExtendSim 8. The book can be used as either a desk reference or as a textbook for a course in discrete event simulation. This book is intended to be a blend of theory and application, presenting just enough theory to understand how to build a model, design a simulation experiment, and analyze the results. Most of the text is devoted to building models with ExtendSim 8, starting with a simple single-server queue and culminating with a transportation depot for package transfer and delivery. I have built all the models contained in this book with ExtendSim 8 LT, which limits the number of modeling blocks, but otherwise has the required ExtendSim 8 capabilities. ExtendSim 8 LT is not included in this book. ExtendSim 8 is a trademark of Imagine That, Inc.