For the fabrication of microstructures with minimum lateral dimensions in the micrometer range and structural heights of up to several hundred micrometers a new microfabrication technique has been developed by the Karlsruhe Nuclear Research Center. The so-called LIGA method is based on a combination of deep-etch X-ray-lithography, electroforming and plastic molding. High-quality micromold inserts can be prepared by deep-etch X-ray lithography and electroforming allowing an arbitrary choice of cross section of the microstructures. By optimization of the process parameters of the reaction injection molding process using casting resins on a methyl methacrylate base it has been shown that a yield of 100% can be achieved. For molding microstructures without any flaws, the mold material has to be degassed, the cavity of the mold must be evacuated and the shrinkage due to polymerization has to be compensated by applying a holding pressure. With an internal mold release agent which is a special salt of an organic acid, cycle times of about 12 min have been achieved in the micromolding process step which are very much shorter than the exposure and development times in the direct lithographic production of microstructures. Such plastic structures can represent the final product or, if microstructures are to be made of metal, they are used as templates in a subsequent electroforming process. Depending on the geometrical configuration of the microstructures, either a metallic gate plate or an electrically conducting plastic layer are used as the electrode for electroforming.