This paper reviews studies of particle concentrations and sources in homes and buildings. Three major studies of indoor and outdoor concentrations in U.S. homes are summarized in detail and compared. A number of smaller studies in homes are also briefly summarized. One study of 38 buildings and several smaller studies of buildings are also reviewed. All major studies found that an important indoor source of fine and coarse particles was cigarette smoking, with estimated increases in homes with smokers ranging from 25 to 45 µg/m3 PM2 5. Several studies identified cooking as a second important source. All of the three major studies found a substantial portion of indoor particles to be due to unexplained indoor sources, suggesting a topic for future research. One study also found a large unexplained increase in personal exposure to PM10 compared to concurrent indoor or outdoor concentrations—the so-called "personal cloud," a second topic for further research. The impact of outdoor air particles on indoor concentrations was considered. Equilibrium mass balance models indicate that this is a function of the penetration P through the building envelope, the air exchange rate a, and the particle decay rate k. EPA's PTEAM Study provided an estimate for k of 0.39 h 1 for fine particles and 1.01 h 1 for coarse particles. Two studies suggest that P = 1 for PM10. Using these results, the fraction of outdoor air PM2 s and PM10 particles found indoors at a given air exchange rate can be calculated. The protective effect of reducing air exchange rates during periods of high outdoor particle pollution can thus be quantified. Most studies showed poor correlations of personal exposures with outdoor concentrations. However, for those studies with seven or more repeated measurements, longitudinal regressions on exposures of individual subjects showed much improved correlations with outdoor particle concentrations. This new finding provides some support for the epidemiological findings of increased morbidity and mortality associated with higher outdoor particle concentrations.