There is something magical about ''round birthdays.'' They make one stop and think about where he is, how he got there, and where he is going. The same is apparently true of conferences, especially those like ICPEAC which represent the periodic coming together of a broad range of scientists in a reasonably well-defined discipline. This Vienna conference is the 20th in the ICPEAC series, and a retro- and pro-spective analysis seems appropriate. At the first ICPEAC in New York (1958), there were about 50 participants. In Vienna (1997), there were more than 800 participants. How do we account for this growth? ICPEAC is one of the most democratic conferences of its size. For example, input from a general committee of 50 representing the many geographic and sub- disciplinary areas gives continuous refreshment of ideas for the invited program. And, as in any democracy, there is a constant state of turmoil and self doubt. After all, atomic collisions is a ''mature'' field, and was a ''mature'' field at the time of the first ICPEAC in 1958, 50 what can one expect in new developments? This self doubt is best expressed in the comment of Ben Bederson (the ''Father of ICPEAC'') following the second in Boulder, Colorado, in 1961: ''This conference is the second in a series of informal meetings organized by a group of workers in the general field of electronic and atomic collisions. The first such meeting was held at New York University in 1958, and we will probably continue to meet at irregular intervals in the future ... '' As part of my ''keynote'' talk at the New York ICPEAC XVI in 1989, I gave a brief history of the conference which we can build on to round things off for our twentieth.