I used to hate it/be terrified of it, and then I read on a blog I followed that the person underlined a lot in her favourite books. I'd never really liked the thought, but the more I read, the more I fell into the underline camp. I usually do it with a pencil, though, and if I want to underline a whole paragraph I just put a line beside it instead of underlining all of it (seems like a waste). I usually underline beautiful phrases/metaphors, and whatever is uniquely and especially well articulated. Recently, I was reading The Hours, which is a very light book to read, but is also a dense net of references to Virginia Woolf's books. Read and re-read, it's quite rewarding (as is VW, though in a different way), and I really wanted to outline a bunch of things but it's not my copy :( When studying, I definitely underline. It actually helps me focus, too, and when I have to re-read for exams I know that the key points are already underlined.