Within the class of existing or conceived photochemical air quality simulation models (PAQSM's) there are only several basic approaches. The United States Environmental Protection Agency (U.S. EPA) in the mid-1970's reviewed the various urban scale models which were in existence and chose three, embodying distinct approaches, for further refinement, development and evaluation. The purpose of this paper is to describe the PAQSM's emerging from research and development projects of the U.S. EPA. The presentation will not attempt a detailed discussion of chemical mechanisms or numerical schemes. Rather, emphasis will be placed on the models' basic structures, data requirements, computer requirements, and problems encountered in applying them. Also, two major field programs will be described which have been conducted by EPA to support testing and evaluation of the models.