The XXth General Assembly of the International Astronomical Union was held in Baltimore, Maryland USA from August 02 to 11, 1988. The Inaugural Ceremony on August 02 was held in the presence of representatives of the United States Governn:ent, t~e S~ate of Maryland, the City of Baltimore and the host institution -the Johns Hopkins Umverslty- as well as of the National and Local Organising Committees. The scientific programme maintained the high standards of the Union and the scientific proceedings may be found either in this volume or in volume 8 of Highlights of Astronomy. The scientific programme was organised by the 40 Commission Presidents and coordinated by the General Secretary (1985-1988), Dr. J.-P. Swings. The local arrangements were effectively made through the National Organising Committee under the Chairmanship of Prof. F. Drake and the Local Organising Committee under the co-Chairmanship of Prof. A. Oavidsen and Dr. R. Giacconi. The smooth day to day operation of the meeting resulted from the incomparable dedication of Karen Weinstock and Harold Screen.

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I Inaugural Ceremony.- Address by Dr. A. Davidsen, co-Chairman of the Local Organizing Committee.- Address by Dr. R. Giacconi, co-Chairman of the Local Organizing Committee.- Address by Dr. F. Drake, Chairman of the National Organizing Committee.- Address by Dr. S. Muller, President of the Johns Hopkins University.- Address by K.L. Schmoke, Mayor of the City of Baltimore.- Address by M.A. Steinberg, Lieutenant Governor of the State of Maryland.- Address by Dr. F. Drake, on behalf of Dr. B. Shakhashiri, Assistant Director for Science & Engineering Education, National Science Foundation.- Address by Dr. N.W. Hinners, Associate Deputy Administrator, NASA Headquarters.- Address by Dr. W. R. Graham, Science Advisor to the President & Director of the Office of Science & Technology Policy.- Address by Prof. J. Sahade, President of the International Astronomical Union.- II Extraordinary General Assembly.- 1. Extension of the membership of the Executive Committee by a President-Elect.- 2. Establishment of a new category of adherence to the Union, Associate National Member.- III Twentieth General Assembly.- Session 1: 2 August, 1988.- Session 2: 11 August, 1988.- IV Report of the Executive Committee.- Executive Committee 1985-1988.- 1. Introduction.- 2. Administration.- 3. Commissions of the IAU.- 4. Scientific Meetings.- 5. Publications.- 6. Relations to other organisations.- 7. Financial matters.- 8. List of Adhering Organisations.- 9. List of deceased Members.- 10. List of IAU Publications.- V Report of Meetings of Commissions.- 4 Ephemerides (Ephémérides).- 5 Documentation & Astronomical Data (Documentation & données astronomiques).- 6 Astronomical Telegrams (Télégrammes astronomiques).- 7 Celestial Mechanics (Mécanique céleste).- 8 Positional Astronomy (Astronomiede position).- 9 Instruments & Techniques.- 10 Solar Activity (Activité solaire).- 12 Radiation & Structure of the Solar Atmosphere (Radiation & structure de l'atmosphère solaire).- 14 Atomic & Molecular Data (Données atomiques & moléculaires).- 15 Physical Study of Comets, Minor Planets & Meteorites (Etude physique des comètes, des petites planètes & des météorites).- 16 Physical Study of Planets & Satellites (Etude physique des planètes & des satellites).- 19 Rotation of the Earth (Rotation de la Terre).- 20 Positions & Motions of Minor Planets, Comets & Satellites (Postitions & mouvements des petites planètes, des comètes & des satellites).- 21 Light of the Night Sky(Lumière du ciel nocturne).- 22 Meteors & Interplanetary Dust (Météores & poussière interplanétaire).- 24 Photographie Astrometry (Astrométrie photographique).- 25 Stellar Photometry & Polarimetry (Photométrie & polarimétrie stellaire).- 26 Double & Multiple Stars (Etoiles doubles & multiples).- 27 Variable Stars (Etoiles variables).- 28 Galaxies.- 29 Stellar Spectra (Spectres stellaires).- 30 Radial Velocities (Vitesses radiales).- 31 Time (L'heure).- 33 Structure & Dynamics of the Galactic System (Structure & dynamique du système galactique).- 34 Interstellar Matter (Matière interstellaire).- 35 Stellar Constitution (Constitution des étoiles).- 36 Theory of Stellar Atmospheres (Théorie des atmosphères stellaires).- 37 Star Clusters & Associations (Amas stellaires & associations).- 38 Exchange of Astronomers (Echange d'astronomes).- 40 Radio Astronomy Radioastronomie.- 41 History of Astronomy (Histoire de l'astronomie).- 42 Close Binary Stars (Etoiles doubles serrées).- 44 Astronomy from Space (L'astronomie à partir de l'espace).- 45 Stellar Classification(Classification stellaire).- 46 Teaching of Astronomy (Enseignement de l'Astronomie).- 47 Cosmology (Cosmologie).- 48 High Energy Astrophysics (Astrophysique des hautes énergies).- 49 The Interplanetary Plasma & the Heliosphere (Plasma interplanétaire & héliosphère).- 50 Protection of Existing & Potential Observatory Sites (Protection des sites d'observatoires existants & potentiels).- 51 Bioastronomy: Search for Extraterrestrial Life (Bioastromie: recherche de la vie dans l'Univers).- VI Functions and Statutes of the Union Pink/FSl.- 1. Useful addresses.- 2. History of the Union.- 3. IAU Representation to other Organisations.- 4. Services of the Union.- 5. Rules for Scientific Meetings.- 6. Statutes, By-Laws & Workings Rules.- VII Membership Green/Ml.- 1. List of Adhering Countries.- 2. Membership of Commissions.- 3. Geographical Repartition of Members.- 4. Alphabetical List of Members.- VIII IAU Style Book S1-S50.