Converting energy from the sun's radiation into electrical current has been a reality for over 40 years but the efficiency derived from these devices has been low and not economically practical. With recent developments in solar cell technology, including multi-junction cells, conversion efficiency of nearly 40% has been demonstrated in the laboratory. The efficiency gain is due to the structure of the cell coupled with optics used to concentrate the sunlight onto the device. The concentrator design requires that the cell be uniformly illuminated to achieve the highest efficiency. Optical analysis software is used in the design and simulation of the system comprised of the solar radiation, optical concentrator and solar cell. This paper will describe the modeling of these concentrators and illustrate how the simulation can provide improved designs to achieve high illumination uniformity.