In the last few years molecular dynamics has become a valuable tool in calculating the thermodynamical, structural and transport behaviour of a lot of many-particle systems in different fields of science and application. First the aims and essential features of molecular dynamics (MD) are described. In the second part self-diffusion coefficients of N2/CH4mixtures for isotope substituted methanes in dependence on temperature, pressure, mass and diameter are calculated. Avoiding large computational efforts the calculations are performed by a kinetic hard sphere theory fitting the hard sphere diameters by MD runs.