The following driving forces of smoke movement in buildings are discussed: stack effect, buoyancy of combustion gases, expansion of combustion gases, wind effect, and elevator piston effect. Based on an analysis of elevator piston effect, it is concluded that the likelihood of smoke being pulled into an elevator shaft due to elevator car motion is greater for single car shafts than for multiple car shafts. Methods of evaluating the location of the neutral plane are presented. It is shown that the neutral plane between a vented shaft and the outside is located between the neutral plane height for an unvented shaft Äequation (23)Å and the vent elevation. Other cases demonstrated that elevated temperatures of combustion gases can result in downward smoke flow from one floor to another. Much of the information in this paper is applicable to the migration of other airborne matter such as hazardous gases and bacteriological or radioactive matter.