When, if ever, do you tell your employer you're interviewing somewhere else?

I've been looking into making a potential transition from my current company, and have started the interview process at a few places. Since this is my first time making a transition like this, I'd love to know what others' experiences were. Over the past few quarters I've exceeded expectations, but have found fairly limited opportunities to continue career growth, which is why I'm looking to move on. Should I be completely up front with my current employer right now, or wait until I have an offer in hand to start that conversation?