FEDERAL MARGIN REQUIREMENTS constitute a significant refinement in the evolution of Federal Reserve policy. Yet, after almost twentyfive years of experience and increasing respectability of the instrument, a thorough treatise devoted specifically to it had not appeared. This study undertakes a broad treatment of the subject. Conceptual aspects and controversies are first examined. The mechanics and measurement of stock-market credit are explained, as are resources for analysis of margin-trading activity. The economic and historical background of margin regulation and prior efforts to control stock-market credit excesses are reviewed. The legislative history of margin regulatory authority is traced, and the implementation through the technical formulation and adaptation of Regulations T and U is detailed. Policy is approached chronologically, with each Federal Reserve action discussed and appraised in its particular economic setting. A serious effort is made to analyze and determine some of the more significant relationships between margin requirements and, successively, margin-trading activity and stock-market credit volume, common-stock prices, share-trading volume, capital markets, and organized processes in the secondary marketing of securities. Finally, certain deficiencies and limitations on margin regulation deserving of further or continuing attention are noted. Margin requirements, in regulating the terms of transactions in securities, continue to be a subject of some controversy. A majority of economists, perhaps, view the selective concept-particularly margin requirements-as a useful addition to resources of monetary authorities. Major criticism centers around contentions that the techniques are both ineffective and authoritarian. Supporters generally view selective regulations as supplementary to other instruments, not substitutive, and of only limited usefulness in the management of general economic tendencies. Stock-market credit, arising from net flows of payments between cash and margin traders, is measured by several statistical series. But there are serious shortcomings in the measures, and the resources for examining margin-trading activity are even more deficient.