Air quality models for ozone have been under development and in operation for over twenty years in the United States. The authors are presently in the third generation of such models. The defining feature of third generation models is that they are fully multi-scale, able to simulate the relevant processes leading to ozone accumulation on scales ranging from point source size to continental. The key feature of multi-scale models is the ability to nest subdomains of finer grid resolution inside domains of coarser resolution, providing an appropriate level of detail for different portions of the modeling domain.