Infiltration is the (usually uncontrolled) flow of air through leaks in the building envelope, driven by natural and mechanical pressures. Before the oil crises, there was not a lot of interest in infiltration. For houses and other envelope-dominated buildings, however, infiltration typically accounted for all of their ventilation needs and 1/3-1/2 of their space-conditioning load. Starting in the mid-70s there was a realization that this important problem was not well understood, but represented an important energy-saving opportunity. Research institutions around the world began significant R&D efforts on it; the two most prominent in the U.S. were Princeton and LBL. This seminar will cover the main scientific questions and results in the field of infiltration with emphasis on work done at LBL. The talk will broadly cover the themes of air change rate measurement and interpretation techniques, modeling of leakage and associated measurement techniques; simplified physical modeling of infiltration and related indoor air quality; characterization of the US housing stock; ventilation and energy impacts of infiltration; ventilation strategies and standards, and combined heat and mass transport within building envelopes.