If you choose one-sided decals, you will want to put them on the outside of the glass so that people passing by can see. However, if you order a double-sided vinyl decal, then you can put it on the inside. This way, everybody can see the decal through the transparent glass, but it'll be protected from the elements and people when it is on the inside. This applies to all types of window decal printing - truck decals stickers, car decals, etc.