Well-defined single crystals of high quality are of importance in some branches of industry as well as in fundamental investigations of materials research. This monograph is devoted to the growth of crystals using optical furnaces. Optical furnaces can be understood and designed making recurrence to concepts that stem from analytical geometry. Therefore, these ideas are presented taking both "faces" of analytical geometry into account. For the operation and maintenance of optical furnaces it is advantageous to gain an understanding of their principle of operation and alignment sensitivities. The method of analysis presented in this book strongly relies on pictorial representations and ray tracing is used as a means to visualize the working principles of these furnaces.