This book is written to assist engineers who are working to design, improve, or optimize chemically reacting flow processes. While the general subject of chemically reacting flow is quite broad, our intent here is rather focused. We are particularly concerned with laminar, internal flows, often with heterogeneous chemistry at the reactor surfaces. The emphasis is motivated by applications in thin-film processing and combustion. Nevertheless, since the development in the book is grounded in fundamental principles, the reader should be well prepared to extend the concepts into new applications. We have intended to write in a form that can be used either as reference material or in an educational setting.