The first four sections in this chapter deal with general information on optical principles and heat transfer that is important in concentrating collectors. The next is concerned with arrays of cylindrical absorbers over diffuse reflectors. The following three treat the performance of compound parabolic concentrator (CPC) collectors. The balance of the chapter is concerned with imaging collectors and discusses linear collectors of types being used in experimental industrial process heat and pumping applications and multiple heliostat ‘‘central-receiver’’ collectors.