If you have a previously papered surface, you should strip the wallpaper. This may be essential depending on what kind of wallpaper you currently have and how you want to refinish the walls.

There are some conditions under which you may decide not to remove existing wallpaper. You can safely paper over a single layer of smooth, well-adhered wallpaper if it's free of bubbles, buckles and wrinkles. You'll need to do some prep work including repasting any loose paper, patching as necessary, cleaning the walls and applying a special primer.

Installation over existing paper isn't recommended if:

You suspect there may be more than a single layer of wallpaper on the surface.
The existing wallpaper has a vinyl, foil or plastic film. In this case, the wallpaper should be removed even if there's only one layer in good condition. Wallpaper adhesive won't adhere well to these surfaces.
You're planning to hang vinyl wallpaper. If the previous wallpaper is also vinyl, the glue between the two papers won't dry well. If the old wallpaper isn't vinyl, the glue under it may get wet and start to mildew.
The existing wallpaper is grass cloth, burlap or cork.
You can hear crinkling sounds when running your hand over the surface of the wall. If you do, the wallpaper has buckled and it needs to be removed.
If there are tears in the wallpaper at the corners of the walls; when walls settle, wallpaper sometimes tears in a corner.
If you're planning to paint, you must remove the wallpaper. Paint will seal the surface and make the wallpaper almost impossible to remove in the future. Also, the painted surface won't look as smooth as it could and your efforts will be wasted.

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