Publisher Summary This chapter examines the motion of a single aerosol particle in a fluid. A particle in stationary air would settle under the action of gravity and reach a terminal velocity quite rapidly. The settling velocity is defined as the terminal velocity of a particle in a still fluid. Because the particle's velocity does not change once it reaches the settling velocity, the acceleration on the particle is zero at this velocity. Therefore, the net force on the particle must also be zero. Assuming that the only forces on the particle are the aerodynamic drag and gravity, then for a solid, nonrotating, and spherical particle, only a vertical drag force would be present that must balance gravity. For very small particles, collisions with randomly moving air molecules would cause the particle to undergo a nondeterministic random walk called Brownian motion. Besides diffusion and gravitational settling, a third mechanism that can cause inhaled pharmaceutical aerosols to move relative to the fluid and deposit on the walls of the airways in the respiratory tract is particle inertia. In particular, if the fluid travels around a bend, a particle that is massive enough may not be able to execute the bend and would deposit on the wall. The deposition of particles in this manner is called inertial impaction.