"The Optics" was a product of Kepler's most creative period. It began as an attempt to give astronomical optics a solid foundation, but soon transcended this narrow goal to become a complete reconstruction of the theory of light, the physiology of vision, and the mathematics of refraction. The result is a work of extraordinary breadth whose significance transcends most categories into which it might be placed. It gives us precious insight into Kepler's thought during this crucial period, an insight all the more valuable in that most of his working papers from that time have been lost. Second, it is the culmination of a long and rich tradition in the science of optics, in distinct contrast with the new optical thought represented by Descartes. And third, it presents discoveries in the physiology of vision, photometry, and the geometry of conic sections which have become part of our intellectual heritage. Especially notable are Kepler's discovery of the inverted retinal image, his theoretical grounding of the inverse-square photometric law, and his insights into the relations between the various conic sections.