Although peer-to-peer networking applications continue to increase in popularity, there have been few measurement studies of their performance. We present the rst study of the locality of les stored and transferred among peers in Napster and Gnutella over month-long periods. Our analysis indicates that the locality of les is skewed in all four cases and ts well to a logquadratic distribution. This predicts that caches of the most popular songs would increase performance of the system. We also took baseline measurements of le types and sizes for comparison over time with future studies. Not surprisingly, audio les are most popular, however a signi cant fraction of stored data is occupied by videos. Finally, we measured the distribution of time peers in Gnutella were available for downloading. We found that node availability is strongly inuenced by time-of-day eects, and that most user's tend to be available for only very short contiguous lengths of time.