The XIIlth General Assembly of the International Astronomical Union, held in Prague from 22 to 31 August, 1967, brought together more than 1800 active astron­ omers from 40 countries. The agenda of the Assembly ranged from administrative questions to highly intricate scientific problems. Thirty-eight Presidents of the IAU Commissions contributed by their reports to a detailed survey of Astronomy for the period 1964-67. These reports were published in the Transactions of the IAU, Volume XIIIA. Transactions volume XIIIB bring the results of the administrative meetings and of the meetings of Commissions held during the General Assembly. The present volume is devoted to the most important scientific results of the General Assembly as presented in the Invited Discourses, Joint Discussions, and at Special Meetings. It is an established policy of the Union to invite prominent astronomers to hold, during the General Assembly, formal discourses on topics of major importance due to recent significant developments. These Invited Discourses cover, as a rule, broad fields of astronomical interest and are meant to give basic information also to scien­ tists from other branches. Subjects which call for a more detailed technical approach are dealt with in the Joint Discussions or at Special Meetings. Here, invited speakers present authoritative surveys complemented by communications on current or envisaged projects. The proceedings of two other meetings, on Moon Probes and on Coordination of Solar Observations Made at Ground-based Observatories and with Space Vehicles, are of a sufficiently wide interest to warrant the inclusion into this volume.