Hi! As others have stated, "Black Tie Invited" means that your hubby should wear a formal suit with a tie (matched coat and slacks with a white or off-white long-sleeved button-down dress shirt and conservative tie). There is no need for him to rent/buy a tux, but he will not be out-of-place if he wants to do so. It also means that you must wear a dressy outfit. Most womens' pantsuits (or even business suits with a skirt) will not be appropriate for this occasion. You do not need a buy a gown, but you will need to pull out a dressy dress from your closet. If you have a floor-length dress in the closet, this is the time to wear it!