I'm curious on this and I've been thinking about it lately. I believe they exist - after all, our universe is so huge, it'd be a bit presumptuous to say we must be the only life form on it. Portrayals in books/movies seem to fall into three categories: 1) a deadly bacteria, virus or other type of invisible organism, 2) shape-shifters of some kind, or 3) humanoid-shaped "gray alien" creatures. I certaintly wouldn't expect they'd look like us - or just like us except with gray skin, black eyes and larger heads. Soooo..what are your thoughts?