Traditional microeconomic theory is concerned with the allocation of scarce resources through the mechanism of prices and markets. The efficient allocation of resources by prices and markets is called allocative efficiency. In emphasizing the allocative function of prices and markets, microeconomic theory has ignored the allocation of resources within firms, and instead assumes that firms are always internally (X) efficient, cost minimizers. X-efficiency theory shows that protection from competitive pressures produces not only allocated market-inefficiency, but inefficiency within the firm. This book is the most current, in-depth, and comprehensive review of X-efficiency theory, especially as it relates to regulatory economics and policy. It provides an understanding of X-efficiency by developing the theory, exhibiting empirical evidence, and, finally, reviewing applications of the theory.