This book is an up to date, introductory treatment of geometrical optics, from the point of view of the future optics practitioner. The principles of gaussian optics and first-order layout and design are emphasized, based on the tracing of two paraxial rays and the associated optical invariant. The radiometry of lens systems is seen to rest on the same concepts. Third order aberration theory is developed in detail. Complete examples of third order design are provided, together with software tools that allow the reader to follow the examples in detail, or to develop other examples independently. Throughout the book, the understanding of fundamental principles is given preference over the mere following of recipes. Several problems at the end of each chapter allow the student to practice and extend the concepts taught. In addition to Gaussian optics and aberrations, the basic principles of several types of optical instruments and components are treated, including gradient index and diffractive optics. A complete yet simple treatment of gaussian beams is included, that is also based on two paraxial rays and the optical invariant. The book concludes with an outline of the optical design process and solved design problems.