Licensing is the principal means of accessing the use of patented technology, and it occurs under terms that are infinitely varied and complex and whose effects are not straightforward. Thus, whether a patented upstream invention is available for licensing—and under what conditions—is possibly the principal determinant of whether exclusive rights can impede or alternatively, facilitate the conduct of follow-on research. Unfortunately, data on licensing are very limited for two reasons. First, unlike patents, government administrative processes generate information on only some licenses whose representativeness is unknown and for which detailed information may be lacking and not publicly available. Licenses to or by