This chapter reviews elements of aerosol dynamics and physics and focuses on aerosol flows of spherical, single-species particles, although we discuss extensively power-law agglomerates. Coagulation and agglomeration modify the size distribution without changing the mass concentration, whereas gas-to-particle conversion modifies the aerosol mass concentration. Internal Processes M. Smoluchowski presented the first treatment of the Effect of particle coagulation on the discrete particle size distribution. Although his derivation referred to hydrosols, the derivation and the final equation are equally applicable to aerosols. In the aerosol physics literature, it is frequently assumed that transport mechanisms act independently of each other, an assumption that usually leads to fairly good results. The coupled calculation of diffusion and inertia in an Eulerian description is important, because these two mechanisms act in unison in many aerosol processes, for example, filtration and sampling. Most numerical techniques developed to solve the general dynamic equation neglect spatial inhomogeneities by assuming that the aerosol is spatially well mixed.