In conducting an emission inventory, sources may conveniently be divided into two major classes, specific sources, largely industrial, which are studied on an individual basis and multiple sources, those too numerous to be treated individually. This latter class includes automobiles, trucks, residences, apartments, incinerators and many other sources which collectively contribute significantly to urban air pollution. The following discussion will be limited to a description of the air pollution emission inventory presently under development as part of the Special Air Pollution Study of Louisville-Jefferson County, Kentucky.