I've seen several agents say that they groan at the sight of prologues. This has led to an avalanche of advice that you shouldn't write them. But I don't necessarily agree. While I do think that prologues are overused and should be well considered as part of a story's structure, I don't think they should be gotten rid of all together. They have a place in some stories.
It's not prologues that are the problem. Unnecessary prologues are the problem. For a long time they were pretty much considered part of the fantasy genre's story structure (others, too, but fantasy was hit hard) and once it becomes something you feel you have to write, it loses its purpose and usage and becomes a dumping grounds. Often, because they are considered a necessary part of the book, they're nothing more than a first chapter under another name.
Traditionally, prologues are literary devices which call attention to the theme of the story or play, or introduce a work. They might be used to introduce a narrator or reveal the thrust of the story. A prologue isn't meant to be a chapter, it's not meant to be a part of the story, it is meant to be the story, in miniature. It's meant to encapsulate the story, to give insight into it and illuminate it, without revealing it entirely.
Prologues have also been used to entice a reader into a story, perhaps by allowing the writer to bring a later section of the story up to the front. Such as when a prologue introduces a character in danger and then the first chapter begins before that event. I think this can be a valid use, but it also has its pitfalls. Putting someone in danger upfront might not matter as much to a reader if we don't know the character, and the first chapter should be able to draw readers in by itself. A prologue used this way can be a crutch.
However, there is another point to be made about prologues. Apparently not everybody reads them. Don't ask me why, I read everything including tables of contents, casts of characters, forewords and prologues. But I know several people who always skip them. *shrugs* I don't get it. But it's got to make things especially confusing in those books where the prologue is actually "chapter one."
Thus prologues present a problem. Nothing should be included that isn't necessary to understanding the story, but necessary information might get skipped if it's in the prologue.
My solution is to write the prologue if it fits, if it speaks to the theme of the story, but isn't necessary to understand the plot. I want a person to be able to read my book from the first chapter to the last chapter and understand everything that happens. However, if they read the prologue, the theme of the story will be more clear from the start. If they don't, the theme may be more cloudy. Theme is a part of story, but it isn't always necessary information. Someone can miss the theme of a book and still understand what's happened and enjoy the story.
For me, prologues are another tool in the box, and I use them when I think I can do it well. What do you think about them? Do you write prologues? If so, to what use do you put them?