do you need wifi or data to use imessage?

I was really nervous to use iMessage because it always charges me whenever i use it.. And I was just wondering if you need wifi or cellular data when you use imessage. I’ve been using iMessage for a day(using wifi) and thankfully my carrier hasn’t charge me for this . But I’m just afraid to use it when I don’t have my wifi or I don’t have my cellular data on . Can I still use iMessage without wifi and not having my cellular data on or do you need wifi or data ?