Johannes Andersen (Denmark) is a former IAU General Secretary and a research astronomer with ~400 professional publications and an IAU Symposium volume behind him.David Baneke (Netherlands) is assistant professor in the history of science at Utrecht University and has written extensively on the history of modern science and scientific institutions.Claus Madsen (Denmark) is a former outreach and policy officer at ESO, a key actor in the preparation of the IYA in 2009, and the author of books on astronomy, including the history of ESO, science communication and European science policy.

• Chapter 1: The Birth of the IAU (1919-1929).- Chapter 2: Crises and Opportunities (1940-1959).- Chapter 3: A Decade of Astronomical Surprises (1960-1969).- Chapter 4: A Two-Faced Period (1970-1989).- Chapter 5: Opening up (1990-2000).- Chapter 6: The IAU in the 21st Century (2001-2019).- Appendix 1: General Assemblies, Membership Numbers, Officials.- Appendix 2: Commission History.- Appendix 3: List of National Members.- Appendix 4: International Schools for Young Astronomers.

Johannes Andersen (Denmark) is a former IAU General Secretary and a research astronomer with ~400 professional publications and an IAU Symposium volume behind him.David Baneke (Netherlands) is assistant professor in the history of science at Utrecht University and has written extensively on the history of modern science and scientific institutions.Claus Madsen (Denmark) is a former outreach and policy officer at ESO, a key actor in the preparation of the IYA in 2009, and the author of books on astronomy, including the history of ESO, science communication and European science policy.

• Chapter 1: The Birth of the IAU (1919-1929).- Chapter 2: Crises and Opportunities (1940-1959).- Chapter 3: A Decade of Astronomical Surprises (1960-1969).- Chapter 4: A Two-Faced Period (1970-1989).- Chapter 5: Opening up (1990-2000).- Chapter 6: The IAU in the 21st Century (2001-2019).- Appendix 1: General Assemblies, Membership Numbers, Officials.- Appendix 2: Commission History.- Appendix 3: List of National Members.- Appendix 4: International Schools for Young Astronomers.

Johannes Andersen (Denmark) is a former IAU General Secretary and a research astronomer with ~400 professional publications and an IAU Symposium volume behind him.David Baneke (Netherlands) is assistant professor in the history of science at Utrecht University and has written extensively on the history of modern science and scientific institutions.Claus Madsen (Denmark) is a former outreach and policy officer at ESO, a key actor in the preparation of the IYA in 2009, and the author of books on astronomy, including the history of ESO, science communication and European science policy.

• Chapter 1: The Birth of the IAU (1919-1929).- Chapter 2: Crises and Opportunities (1940-1959).- Chapter 3: A Decade of Astronomical Surprises (1960-1969).- Chapter 4: A Two-Faced Period (1970-1989).- Chapter 5: Opening up (1990-2000).- Chapter 6: The IAU in the 21st Century (2001-2019).- Appendix 1: General Assemblies, Membership Numbers, Officials.- Appendix 2: Commission History.- Appendix 3: List of National Members.- Appendix 4: International Schools for Young Astronomers.