Optical imaging starts with geometrical optics, and ray tracing lies at its forefront. This book starts with Fermat's principle and derives the three laws of geometrical optics from it. After discussing imaging by refracting and reflecting systems, paraxial ray tracing is used to determine the size of imaging elements and obscuration in mirror systems. Stops, pupils, radiometry, and optical instruments are also discussed. The chromatic and monochromatic aberrations are addressed in detail, followed by spot sizes and spot diagrams of aberrated images of point objects. Each chapter ends with a summary and a set of problems. The book ends with an epilogue that summarizes the imaging process and outlines the next steps within and beyond geometrical optics.