ASHRAE Research Project 590, Control of Outside Air and Building Pressurization in VAV Systems, was officially begun on September 1,1988, with the following stated objective: To develop a methodology, preferably adaptable to manual calculation and to personal computer calculations, by which to compare operating cost, ventilation air quantity and building pressurization resulting from various means of outside air and return air/relief air control in variable volume air conditioning systems, and to evaluate and compare typical representative systems. Because of the complexity of the problem, it was not possible to develop a totally manual method. However, a very versatile personal computer technique was successfully developed as detailed in this report. The PC program accurately models seven basic types of VAV control systems and is easily extended to many sub-types within each of these seven major configurations.