Flooring Tile - Tiling over existing tile

My current tile is very old and looks terrible. I am guessing it is about 20-25 years old. I am told that I can go over the existing tile, providing it is not coming up in any areas. My threshold is high enough to be a little taller the new floor would be. Is there a reason I can not do this? Do I need to use a special adhesive of any kind?

Thanks.

As long as the tile and grout are not cracked or loose (indicating movement which must be addressed first) and the additional height is not an issue you can go over the old tile. You need to clean it real good first so no residue of grease, dirt, soap, cleaning fluids remain. Use a good modified thinset. For a 12" tile use a 3/8" notched trowel to spread the thinset.

I've tiled over existing tiles in 2 bathrooms 4 years ago and no problems.
I think, more important than a good cleaning is to scratch up the old tile as much as possible. Use a small power sander or belt sander with rough-grit sandpaper (50-60) . The cleaner and rougher the old tile, the better the new tile will stick. I used thinset from HD, I think it was "Flex-bond" it's a thinset that is specifically reccommender for tiling over tile. The company name is "Custom" and I think every HD has them. They have pamphlets at the store about there different thinsets and you can always call the company direct and ask questions (their # is right on the bag).
Works great- just follow their instructions !