This History has its origin in a suggestion, made in September 1990 by former IAU General Secretary Derek McNally, who felt "that a 75 year history of the Union was needed before the col­ lective memory of those who knew the Union before the Second World War vanished. It would then be a preparatory volume to a centennial history in 2019. " Indeed, of those who knew the Union that long ago, few are still with us. Six years ago, at Baltimore on August 2, 1988, listening and reminiscing at the Inaugural Ceremonies of the Union's 20th General Assembly, I realized that it was almost exactly half a century ago that, at the age of 24, I attended the Inaugurations at my "first" Assembly: on August 3, 1938 in Stockholm. Now, in 1994, this is almost 56 years ago, three quarters of the Union's age. Only vague recollections - no better than that -lead me back to this event, just before World War II. And so, this is not a history based on recollection, far from it. Recollection was helpful in that it allowed me, better perhaps than a younger author, to appreciate circumstances under which the letters and reports which form the basis for this History were written. The account is largely based on archival documents, collected from a wide variety of sources.