After a brief historical review, we discuss the fundamen­tal physical laws which govern the behaviour of vacuum and play a role in the technique of evacuation. To this end, it is necessary to have some insight in the characteristics and structure of gases, but also of liquids and solids. The properties of rarefied gases at different pressures and temperatures are quite well explained by the kinetic theory of gases. The basic principles of this theory will be discussed, including topics such as the thermal motion of gas particles, the mean free path, rate of incidence, vapour pressure, evaporation rate etc. Also the empirical gas laws of Boyle/Gay-Lussac, Avogadro and Dalton have their formal basis in the kinetic gas hypothesis. Finally, we will focus on some non-equilibrium properties of gases at high and low pressures (thermal conductivity, viscosity, diffusion). The reasonably good agreement between the kinetic transport laws and experimen­tal practice evidences the validity and useful­ness of the kinetic gas theory.