The Harvard College Observatory plate collection is unique in the world in several aspects. First, it is the largest, with a total of approximately half a million plates. Second, because Harvard set up stations very early in the southern hemisphere, the collection covers the entire sky. In fact, the southern hemisphere coverage is slightly better than the northern. Third, the collection is unique in the length of the time interval over which the plates were taken. The first plates were acquired in the northern hemisphere in 1885, and in the southern hemisphere in 1891. There is a substantial gap in the late 1950s and 1960s, but patrol plates were then taken up to 1989. Fourth, the collection contains a very large number of images of a given object. For B = 15 mag or brighter, from several hundred to a thousand or more images can be found; for B = 17.5 mag, one can locate from a few to several hundred images.