Heterogeneous condensation of water vapour is a conditioning technique to separate submicron particles easily from gas streams. The principle of this method is: The fine particles are covered by a liquid shell by condensation, the mass of the particles and consequently their inertia increases and the separation of the enlarged particles is significantly simplified. In this way it is possible to separate particles with a size down to a few nanometers by using special droplet separators or with standard inertial separators. The paper describes the fundamental processes of heterogeneous condensation, such as the generation of a supersaturated vapour phase, the formation and the growth of droplets. Moreover, the possibilities to condition and separate fine particles by heterogeneous condensation are outlined.