As part of the celebration of PRL’s 50th anniversary, we will be presenting throughout 2008 a series of milestone Letters that made long-lived contributions to physics, either by announcing significant discoveries, or by initiating new areas of research. A number of these articles report on work that was later recognized with a Nobel Prize for one or more of the authors. Starting the week of January 2, we will present a few important Letters from PRL in 1958, and the next week from 1959, etc., continuing up through the year 2000.