Stereolithography is the most widely used rapid prototyping technology. Stereolithography also known as 3-D layering or 3-D printing allows you to create almost any 3-D shape you can imagine in a matter of hours. You can hold a working model in your hands two days later! If it can be drawn in a CAD program, it can probably be created. For an inventor looking to create a plastic prototype of an invention, stereolithography is a fast, easy way to turn CAD drawings into real objects.
A 3-D model of your object is created in a CAD program. Then, software slices your CAD model up into thin layers. The 3-D printer's laser "paints" the layer, exposing liquid photopolymer to a laser beam and hardening it. The layer is on a platform that drops down a fraction of a millimeter and the laser paints the next layer. This process repeats, layer by layer, until your model is complete. You can actually see each layer being built if you stand next to the stereolithograph apparatus (SLA).

Stereolithography is not an inexpensive process. The machines themselves usually cost in excess of $250,000. They have to be vented because of fumes created by the polymer and the solvents. The polymer itself is extremely expensive. A common photopolymer used in stereolithography, typically costs about $800/gallon. For these reasons, it is uncommon to find stereolithography machines anywhere but in large companies.
Stereolithography generally is considered to provide the greatest accuracy and best surface finish of any rapid prototyping technology.