The purpose of the report is to describe in a general way the various facets of a computer program that comprise the urban airshed model. Emphasis was placed on the preparation of an efficient program and on insuring the applicability of the program in any urban airshed. An airshed is essentially defined by its terrain, meteorological, and source inputs. Since these parameters are all treated as input data to the program, no difficulty should be encountered in applying the program in a variety of urban airsheds. The most cumbersome aspect of program usage is the large effort required in the preparation of the meteorological input data. Two programs are used in the Los Angeles Basin simulation effort. The main program, the Atmospheric Pollution Simulation Program, calculates pollutant concentrations as a function of space and time using source and meteorological inputs. The structural and the operational aspects of this program are discussed. Aircraft emissions are treated as input to this program via magnetic tape. The computer program which generates this tape, The Aircraft Emissions Program, is also discussed.