The spatial beam structure that is formed in the laser cavity together with the active element constitute the most critical laser elements. The engineering devices, such as excitation systems, lasing gas circulation systems, and so on, are usually at the top of the agenda of scientific meetings: the divergence problem has never previously been discussed by a broad community of experts. The conversion of scientific and technological experience accumulated during the development of high power military laser, where efficiency was not necessarily the major driving force, into commercially viable industrial lasers, will require attention to be focused on the general problems of beam divergence. This text presents a dialogue between specialists in resonator cavity design from NATO and Cooperating Partner countries. The book is divided into four parts covering: theory, in which fundamental questions of resonator physics and optical design are addressed; theoretical and practical problems; the design of optical elements; and the optimization of designs.