Particles in the atmosphere may be either primary or secondary, solid or liquid. They come into the atmosphere, and leave it again, by a wide variety of routes. After looking at general aspects of particles such as definition of size and the way they behave in the atmosphere, we will describe the sources of particles and methods for controlling emissions. Particles less than 10 m in diameter have very low sedimentation speeds under gravity, and may remain in the air for days before eventually being washed out by rain or impacted out onto vegetation or buildings. They are an important environmental pollutant, being responsible for loss of visual range, soiling of surfaces and health effects on people. Knowledge of their concentrations, size distributions and chemical composition is therefore needed, but for various reasons these are quite hard to obtain correctly. There is an important historical difference in the way that gases and particles have been treated as pollutants. Measurement methods for gases rapidly became specific to the gas, so that concentrations of SO2, NO2 etc. have been quantified. In contrast, the predominant method for particle concentration measurement has been filtration, from which mass loading is identified regardless of chemical composition.