To meet the challenges of foreign manufacturers in the coming decade, American industry needs to increase its productivity markedly. A major obstacle to marketing new products is the time involved in producing the prototype parts required to visualize and test new designs. In many industries prototyping consumes a substantial segment of the development cycle. Several devices introduced recently produce prototypes rapidly, directly from computer files generated by CAD.The most mature rapid prototyping device is stereolithography, which produces a prototype from polymeric materials by photopolymerization using a material additive technique. The prototype is fabricated in a layering fashion, starting from the bottom layer and going to the top layer. The layers are constructed, one at a time, writing the layer description, point by point, by scanning a UV laser over the surface of a liquid photomonomer and curing it to form polymer at each location where the laser meets the liquid. When a layer is formed, the built structure is lowered into the vat of liquid photomonomer a distance of one layer thickness, and fresh liquid is flowed over the top. The next layer is then formed. The laser depth penetration is adjusted to ensure adhesion to the prior layers. Understanding the physical processes involved in the formation process is essential to overcoming some of the physical limitations encountered. Optical diagnostic systems have been designed and used to measure time-dependent thermal profiles and fraction of conversion contours during the polymerization.