This volume assembles papers commissioned by the National Research Council’s Board on Science, Technology, and Economic Policy (STEP) to inform judgments about the significant institutional and policy changes in the patent system made over the past two decades. The chapters fall into three areas. The first four chapters consider the determinants and effects of changes in patent quality. Quality refers to whether patents issued by the U.S. Patent and Trademark Office (USPTO) meet the statutory standards of patentability, including novelty, nonobviousness, and utility. The fifth and sixth chapters consider the growth in patent litigation, which may itself be a function of changes in the quality of contested patents. The final three chapters explore controversies associated with the extension of patents into new domains of technology, including biomedicine, software, and business methods.