An understanding of ambient particle transport from source to receptor requires knowledge about particle penetration across building envelopes. Therefore, it is essential to gain insights into particle penetration in infiltrating air and the factors that affect it in order to assess human exposure more accurately, and to further prevent adverse human health effects from ambient particulate pollution. In this work, the understanding of airborne particle infiltration across leaks in the building envelope was advanced by performing modeling predictions as well as by experimental investigations. The modeling analyses quantified the extent of airborne particle penetration through building cracks and wall cavities using engineering analysis that incorporates existing information on building leakage characteristics, and knowledge of pollutant transport processes. The experimental work involved three distinct settings that represent different scale and increased complexity of air leakage pathways associated with building envelope building cracks, windows, and an entire house. Particle penetration across building cracks, using rectangular slots as surrogates, was determined as the ratio of particle concentration downstream to that of upstream of the slot in a specifically constructed laboratory apparatus. Particle penetration through windows, which were installed in a two-chamber system, was inferred by analyzing data on the dynamic relationship of particle concentrations in both chambers. In a field study, a single-family house, located in Clovis, California, was studied to evaluate particle penetration through the entire building envelope. Throughout the experimental research, the modeling results serve as a baseline for comparison to the data obtained in the experimental studies.