The XXIst General Assembly of the International Astronomical Union was held in Buenos Aires from July 23 to August 1. 1991. The activities of General Assemblies are designed to provide opportunities for scientific exchanges within scientific commission meetings or during informal exchanges and for establishing new personal links between astronomers from different countries and continents, and to review and conduct the administration of the Union. At the Inaugural ceremony on July 23, the Union was honored by the presence of the President of the Republic of Argentina. Dr. Carlos S. Menem, the representatives of the Argentine government, of the city of Buenos Aires and of the National and Local Organizing committees. The excellent scientific programme in Buenoes Aires was organized by the Presidents of the fourty IAU Commissions and coordinated by the IAU General Secretary (1988-1991), Prof. Derek McNally. The local arrangements were taken care of by Dr. Fernando R. Colomb. and his collaborators of the National Organizing Committee and by the Local Organizing Committee under the supervision of its chairman, Dr. Roberto H. Mendez. The present volume, IAU Transactions XXIB. summarizes the work of the XXIst General Assembly. The discourses given during the Inaugural and Closing Ceremonies are reproduced in chapters I & III respectively.

I - Inaugural Ceremony 1991 July 23.- II - Twenty-First General Assembly.- III - Closing Ceremony.- IV - Report of the Executive Committee 1988-1991.- V - Reports of Commissions.- VI Functions and Statutes of the Union.- VII Membership.