Optical lithography is the technology used to create high-density VLSI computer circuit patterns. It is fair to say that small improvements and tighter control of a lithographic process usually lead to improved device yield and device performance. Significant advances in lithography often lead to big competitive advantages for manufacturers. Advancing lithographic technology is not easy. Evaluating engineering proposals is often very expensive. Mathematical modeling of lithography process steps often allows faster evaluation of new lithographic ideas and consequently cuts cost substantially. In this talk I will discuss models and the results of models used to simulate the three basic steps used to create photoresist patterns for VLSI circuits, i.e. (1) the 3D optical imaging and exposure step, (2) the 3D exposure product redistribution by diffusion during the very important bake step, and (3) the 3D dissolution of the exposed material during the development step. A video of the 3D dissolution process computed using the RD3D (Resist Development In 3 Dimensions) algorithm invented by the author will be shown.