Stereolithography is the most widely used rapid prototyping technology. Stereolithography builds plastic parts or objects a layer at a time by tracing a laser beam on the surface of a vat of liquid photopolymer. This class of materials, originally developed for the printing and packaging industries, quickly solidifies wherever the laser beam strikes the surface of the liquid. Once one layer is completely traced, it's lowered a small distance into the vat and a second layer is traced right on top of the first. The self-adhesive property of the material causes the layers to bond to one another and eventually form a complete, three-dimensional object after many such layers are formed.

Some objects have overhangs or undercuts which must be supported during the fabrication process by support structures. These are either manually or automatically designed and fabricated right along with the object. Upon completion of the fabrication process, the object is elevated from the vat and the supports are cut off.

Stereolithography generally is considered to provide the greatest accuracy and best surface finish of any rapid prototyping technology. Over the years, a wide range of materials with properties mimicking those of several engineering thermoplastics have been developed. Limited selectively color changing materials for biomedical and other applications are available, and ceramic materials are currently being developed. The technology is also notable for the large object sizes that are possible.

On the negative side, working with liquid materials can be messy and parts often require a post-curing operation in a separate oven-like apparatus for complete cure and stability.

Recently, inkjet technology has been extended to operation with photopolymers resulting in systems that have both fast operation and good accuracy. See the section on inkjets.

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