Of all the problems which must be solved when designing or using a high power CO2 laser, those dealing with transmissive optical components (output mirrors, lenses, windows, and beam splitters) can be the most troublesome and costly. This paper discusses a number of topics meant to help the user avoid problems and cut expenses. Some practical reasons to choose one substrate material over another are presented. The physical phenomena which degrade performance of an optic when it becomes overheated are examined, and a simplified figure of merit analysis is used to predict which substrate material will perform best in a given application. Examples of common field problems and ways to control these problems are given. Finally, some specifications important to the selection of lenses and beam splitters for lasers are reviewed.