The mass concentrations of airborne particles in the atmospheric, indoor, and industrial environments are regulated by air quality standards. Epidemiological studies show that there are significant positive correlations between particle mass concentrations and adverse health effects. In this context nanoparticles in the air, which are defined as particles with a diameter (Dp) of less than 50 nm or 100 nm for engineered ones, are gaining increasing attention despite a small contribution to the mass of total airborn particles. Contrary to the mass concentration the number concentrations of atmospheric nanoparticles are quite high in most cases. Moreover there is limited toxicological information on nanoparticles, although the deposition rate of nanoparticles in the respiratory region is known to be relatively high. Accordingly there are a lot of debates about what metric is best to depict the size distribution of nanoparticles, number, surface area, or mass. In this paper, we report methods for measurement of nanoparticles on the basis of those metrics. We also report sources of nanoparticle in the environment and occupational settings. The high number concentration of nanoparticles of 20-30 nm modal diameters have been documented at roadsides. Diesel-powered vehicles are major sources of those nanoparticles in the urban atmosphere. Engineered nanoparticles generate in some occupational settings in the handling processes such as bagging and cleaning with vacuum cleaners.