Lithographic processes belong to the most critical steps in the fabrication of microelectronic circuits. Optical projection lithography which transfers the layout of a mask into a photoresist at the top surface of a silicon wafer is still the workhorse of semiconductor industry. Many innovations regarding the optical imaging system and the introduction of new mask technologies have pushed optical projection techniques close to their theoretical limits. Nowadays, optical projection lithography is used to create 45nm features with a wavelength of 193nm. This paper discusses the impact of the mask on the performance of a lithographic process. For large feature sizes the mask can be considered as infinitely thin and its local transmission and phase can be directly derived from the design. For mask features comparable to the wavelength of the used light and below, the mask becomes a complex scattering object which has a pronounced impact on the intensity, phase, and polarization of the diffracted light. The light diffraction from the mask has to be computed by rigorous electromagnetic methods. Several consequences with respect to the choice of most appropriate mask materials and geometries will be discussed.