These Transactions provide a record of the formal organisational and administrative proceedings of the IAU XXVII General Assembly which took place in Rio de Janeiro, Brazil. It includes the admission of new members, the Resolutions passed, an account of the regular Business Meetings of the Executive Committee, the Divisions, Commissions and Working Groups, and gives the detailed status of the Membership of the IAU. 2434 registered attendees from 76 nations enjoyed a rich scientific programme of 6 Symposia, 16 Joint Discussions, 10 Special Sessions, and 7 scientific sessions. During the programme over 700 papers were presented and more than 1500 posters displayed. The Proceedings of the Symposia are published in the Proceedings of the IAU Symposia Series, and those of the Invited Discourses, Joint Discussions and Special Sessions will feature in Highlights of Astronomy, Volume 15. Together with those 7 volumes, these Transactions cover the entire General Assembly.