Given the relative newness of free-space optics (FSO) technology in commercial applications, few standardized metrics exist for comparing the performance of different systems. Our goal here is to explain some of the design issues surrounding FSO systems and to provide sufficient information to allow potential users to evaluate the suitability of a specific FSO system for a particular application. In addition, we attempt to define the realistic performance limitations of FSO on the basis of existing technology and also to set reasonable expectations with regard to atmospheric conditions.