Since 1967, the most prominent events of a General Assembly of the International Astronomical Union have been published in a separate volume. The Highlights 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 texts of the three invited Discourses and of the papers presented during seven Joint Discussion Meetings and eight Joint Commission Meetings.

Preface. Invited Discourses: Gravitational Lensing. Retrospect on Voyager. Observational Problems in Extragalactic Astronomy. Joint Discussions: I. An Overview of the Interstellar Medium. II. Reference Systems: What are They and What's the Problem? III. Results from SOAT and GRO and Other Recent High Energy Astrophysics Missions. IV. Cosmic Background. V. Origin of Stars of Planetary Systems. VI. HIPPARCOS -- an Assessment. VII. First Results from the Hubble Space Telescope. Joint Commission Meetings: I. Rotation of Solar System Bodies. II. Automated Telescopes for Photometry and Imaging, Performance and Results with IR Arrays. III. Atomic and Molecular Data for Space Astronomy: Needs and Availability. IV. A Proposal for an International Antarctic Observatory. V. Late Evolution of Low Mass Stars. VI. Solar and Stellar Coronae. VII. High-Redshift Galaxies. VIII. Archiving of Current Observational Data (including Solar System Data). Special Contributions: Commission 9: Instrumentation. Commission 46: Teaching of Astronomy. Index of Authors.