They're usually used to place images in the background of renders where using a 3d model is impractical. For instance, suppose you want to want to render a landscape with thousands of trees. Each tree will have thousands of polys, possibly tens of thousands. Until you get your hands on Vue5I this is going to mean a big strain on your processing power! So you'd use real models in the foreground but use alpha planes mapped with pictures of trees (real photos or renders)in the background. If you match the lighting you'll never tell the difference but it will render MUCH faster! Hope that helps! - Baz