In searching for a subject for this talk, I find that some addresses of retiring presidents have been discussions of topics relating to the social responsibilities and problems of physicists, while others have been technical reviews of research in a particular phase of physics. Professor Bridgman struck a compromise by giving a retiring address which was part political, part physical, and, being from Harvard, I have naturally decided to follow his pattern. One of the barriers about which I shall speak today is political and macroscopic, the other is physical and microscopic. As you perhaps have guessed, the political barrier phenomenon of which I will speak is that presented by the present policy of the United States regarding visas for visiting physicists from abroad. Because of limitations of time, I shall not discuss the reciprocal problem of the difficulties sometimes experienced by United States citizens in obtaining passports.