So long as the surface you’re applying them to is clean, dry, and smooth, you can apply wall decals indoors or out. Note, however, that most are designed for indoor use, and any decals used outdoors tend to deteriorate more quickly.This is particularly true if they’re exposed to the elements. If you still want to use the decals outdoors, try to find some that are specifically made for outdoor use or made from a very durable material like premium vinyl so they’ll last longer.