I'm a fan of prologues. This debate has been going on since I joined these forums. Slowly, I've come to understand why a prologue should rarely be used. If you're writing a story set in a modern time, but want to write a prologue to show what happened 100/1,000/10,000 years ago, that is back story. Find a way to insert it into the story through setting or dialogue. Currently, I'm of the opinion that prologues should be used to tie books of a series together. A recap of previous events so the reader doesn't have to reread all of the previous books to keep up with the current book.