This paper describes the database on U.S. patents that we have developed over the past decade, with the goal of making it widely accessible for research. We present main trends in U. S. patenting over the last 30 years, including a variety of original measures constructed with citation data, such as backward and forward citation lags, indices of "originality" and "generality", self-citations, etc. Many of these measures exhibit interesting differences across the six main technological categories that we have developed (comprising Computers and Communications, Drugs and Medical, Electrical and Electronics, Chemical, Mechanical and Others), differences that call for further research. To stimulate such research, the entire database-about 3 million patents and 16 million citations-is now available on the NBER website. We discuss key issues that arise in the use of patent citations data, and suggest ways of addressing them. In particular, significant changes over time in the rate of patenting and in the number of citations made, as well as the inevitable truncation of the data, make it very hard to use the raw number of citations received by different patents directly in a meaningful way. To remedy this problem we suggest two alternative approaches: the fixed-effects approach involves scaling citations by the average citation count for a group of patents to which the patent of interest belongs; the quasi-structural approach attempts to distinguish the multiple effects on citation rates via econometric estimation.

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We examine the patenting behavior of firms in an industry characterized by rapid technological change and cumulative innovation. Recent survey evidence suggests that semiconductor firms do not rely heavily on patents to appropriate returns to R&D. Yet the propensity of semiconductor firms to patent has risen dramatically since the mid-1980s. We explore this apparent paradox by conducting interviews with industry representatives and analyzing the patenting behavior of 95 U.S. semiconductor firms during 1979-1995. The results suggest that the 1980s strengthening of U.S. patent rights spawned ''patent portfolio races'' among capital-intensive firms, but it also facilitated entry by specialized design firms.

Evidence on the "funding gap" for investment innovation is surveyed. The focus is on financial market reasons for underinvestment that exist even when externality-induced underinvestment is absent. We conclude that while small and new innovative firms experience high costs of capital that are only partly mitigated by the presence of venture capital, the evidence for high costs of R&D capital for large firms is mixed. Neverthless, large established firms do appear to prefer internal funds for financing such investments and they manage their cash flow to ensure this. Evidence shows that there are limits to venture capital as a solution to the funding gap, especially in countries where public equity markets for VC exit are not highly developed. We conclude by suggesting areas for further research.