Many physical processes are difficult or even impossible to study by conventional experimental methods. As computing power has increased, it has become possible to model some of these processes by sophisticated computer code. In such cases, the code can serve as a proxy for the physical process. As in a physical experiment, one can vary the inputs to the code and observe how the process output is affected. Such studies are called computer experiments and are becoming increasingly popular surrogates for, and adjuncts to, physical experiments. Much of the methodology for designing, modeling, and analyzing computer experiments can be found only in research papers. The goal of this book is to make these methods accessible to a more general audience. To accomplish this, we have tried to keep the mathematics at a level that will be understandable to readers with Masters level training in Statistics. This has been a challenging task. For example, Gaussian process models are a popular way to describe the output of computer experiments, but Gaussian process models are mathematically complex and likely to be unfamiliar to many readers. We provide an introduction to these models and present references for those who wish to study additional mathematical details of such processes. In other chapters, we relegate mathematical details to notes at the chapter end. To make the book more useful for practitioners, we provide software that can be used to fit the models discussed in the book. Instructions for using the software are included in an Appendix. Examples of the use of the software are interspersed throughout the book. As the software is updated, new versions (and an expanded user's manual) will be put on line through the book web site.)