This book is an outgrowth of a short course that the author has presented for SPIE for the past 25 years or so on the fundamentals of infrared sensing. This field spans several technical disciplines, which can leave the beginner with the “where do I start?” question. The selection of the material included here represents those concepts and terminology that a newcomer in the field of infrared systems needs to understand. These include items that were initially confusing to the author, ones that he has found useful in practice, or concepts that are commonly misunderstood. These are explained in the simplest terms that keep the key ideas. The level of mathematics is generally algebra-based. The book contains sample calculations but does not include problem sets. This is a consequence of the book’s origin as a set of short course notes. It is not intended as a stand-alone college textbook but rather as a self-study reference for the beginning systems engineer. The intended audience is a person with a bachelor’s-level training in science or engineering. It is meant to be an introduction, with sufficient detail to enable the reader to make initial back-of-the-envelope calculations and to understand the basic tradeoffs and trends involved.