Book excerpt: This book is intended to familiarize the reader with the method of Gaussian matrices and some related tools of optical design. The matrix method provides a means to study an optical system in the paraxial approximation. This text contains new results such as theorems on the design of variable optics, on integrating rods, on the optical layout of prism devices, etc. The results are derived in a step-by-step way so that the reader might apply the methods presented here to resolve design problems with ease.