Can you lay ceramic tiles over wooden floor?

Most ceramic tiles are laid over a wooden subfloor, except for those on the ground floor of a house with a slab foundation.

I will add to the answer above by saying that you will need to use a polymer-modified thinset morter when installing ceramic tiles over wood. Most tile-setting morters available today are "modified", but check to be sure, because regular morters are designed to be used over concrete and will not adhere properly to wood.

Also, if the floor area is large and there is more than a tiny bit of flex in the floor, you might end up with cracked tiles, especially if you or your family or guests are heavy. To avoid this, it might be necessary to build up the thickness of the floor by adding another layer of 3/4" plywood, or by using the older method of wire mesh and concrete over the existing wood (in which case you would not need a modified morter).