This book discusses the development of Christiaan Huygens' wave theory, reconstructing the winding road that eventually led to Trade de la Lumiere. For the first time, the full range of manuscript sources is taken into account. In addition, the development of Huygens' thinking on the nature of light is put in the context of his optics as a whole, which was dominated by his lifelong pursuit of theoretical and practical dioptrics. In so doing, this book offers the first account of the development of Hygens' mathematical analysis of lenses and telescopes and its significance for the origin of the wave theory of light. Students of the history of optics, of early mathematical physics, and the Scientific Revolution, will find this book enlightening.