Many countries would love a national firm as successful as Microsoft. Yet the United States has time and again broken up its highly successful companies, as it did with Standard Oil 90 years ago and AT&T 15 years ago (and almost with IBM in the 1970s). Now U.S. v. Microsoft is underway, and the AEI-Brookings Center for Regulatory Studies has provided a signal service by inviting leading economic witnesses on both sides to state their cases and critique each others' cases. A mixture of law and economics, the book's focus is not on the consequences of a possible breakup of Microsoft-the remedy proposed by a judicial finding that the firm violated U.S. antitrust laws-but on whether its marketing behavior harmed consumers. For this reviewer, the United States has the better case, but readers should form their own judgments. An excellent illustration of how thoughtful analysts and the American judicial system accept the importance of preserving healthy competition within the American economy.