While you don’t have to ask for Pop’s permission to marry his daughter (that age-old idea is quickly becoming obsolete), most parents would appreciate a heads-up. Live nearby? See if you can drop by their house or take them to lunch (or dinner, if spending a night with your future in-laws isn’t too much torture). Live far away? A phone call will do. After exchanging the prerequisite pleasantries, cut to the chase: You love their daughter and you’ll be asking her to marry you. They should be thrilled. If not, they suck, and that’s not your problem. If your girlfriend isn’t close with her parents or you can’t trust them to keep your gigantic secret, rest assured it's okay if they find out after the fact.