Designing Kitchens and Bathrooms - What's Under That Tile??

Hi all

I just finished tiling my bathroom floor in 12" ceramic and I'd now like to consider carrying on and doing the tub enclosure walls. My quesion is, what would likely be under the old existing 4" tile and will I need to install new wall board / cement board. Our home is 18 yrs old and I have no idea what they installed tile over in those days - did they use a cement board or did they go right over the sheet rock.
Any advice or comment would be appreciated. And yes, I will truley never know what is under there until I start removing the old tile but I thought I would go out and ask for a little experienced feedback anyway. Based on the age of the home , maybe someone will have some idea....
Thanks

Could be just about anything and you wont know until you get into it. Could be regular drywall, could be greenboard, could be cement board, could be etc. Not likely youll be able to reuse it anyway.:)

I was afraid of that - sounds like floors are so much easier. Thanks for the feedback - I'll now have to investigate the scope of this job knowing that I have to tear out walls and re-install.
Thanks

Most likely, what was under the other walls is also what is under the tile in the shower. As stated above, could be greenboard, could be cement board, could be drywall. What is the price range of the house?? More expensive may suggest better tile materials, but not necessarily.

Thanks for the replys - actually Just Bill, the house would be considered mid price point 1600 sq/ft multilevel, typical burbs neighbourhood - the price is only relevant if you knew the market in this area and what to compare it to.
I would assume new home construction 20 yrs ago would have used plain drywall so the stuff that is there is likely not reusable. Unfortunately, I do not have any dry wall skills so I'll if I want to do this job, I'll have to get some help. I was hoping I could get away with using the original drywall since there are no leaks and the board should be as new. I guess the biggest issue is removing the old tile and thinset without destroying the wall - not possible right?
Thanks again!

Murphys law dictates that tile you want to stay in place will fall off the wall, tile you want to come off will require a bulldozer to move. Yes, it is unlikely that you will get the tile off without damaging the wall underneath.