Since 1967, the most prominent events of a General Assembly of the International Astronomical Union have been published in a separate volume. The "Highlight of Astronomy" (Volume 9) reports on the major scientific presentations made at the XXIst General Assembly July 23 August 1, 1991, Buenos Aires, Argentina. The present volume contains the text of the three invited Discourses and of the papers presented during seven Joint Discussion Meetings and eight Joint Commission Meetings.