The scientific meetings of the !AU are the !AU Symposia, !AU Colloquia, Regional Meetings in Astronomy held under the auspices of the !AU, the Commission Meetings, which may be Joint Meetings of Commissions, and !AU Joint Discussions. This variety means that almost any demand for a meeting can be provided for by the !AU structure, pro vided it is international in character, whatever may be the preferences of the proposers. The !AU General Assemblies include Co~ssion Meetings and Joint Discussions arranged by Commissions, together with the Invited Discourses, and it is these meetings that feature in this volume. The administrative work of the General Assembly is reported in the Transactions of the !AU and the Proceedings of the !AU Symposia, held in the host country or neighbouring countries during the year of a General Assembly, appear in the !AU Symposium Volume series. !AU Collo quium volumes are published separately. The Invited Discourses are a popular feature of the !AU General Assemblies and each of the three discourses presented at the XVIIth General Assembly in Montreal in 1979 appear in this volume. 1979 was the Centenary Year of the birth of Albert Einstein and the occasion was marked, for the !AU, by a discourse of great distinction from Professor S. Chandrasekhar of the University of Chicago, whose contri butions to astrophysics themselves cover fully a half-century. The Invited Discourse by G. Herzberg of the National Research Council of Canada, a Nobel Prizewinner, was equally notable as an !AU occasion.