In this thesis the evaporation from and condensation on a plane liquid surface have been studied by analysis and molecular dynamics simulations. The effect of the condensation coefficient on the inverted temperature gradient for a two-surface evaporation-condensation geometry is investigated by the moment method. The influence of the molecular exchange phenomenon on the gas-kinetic treatment of evaporation and condensation is shown to be neglible under certain assumptions. Methods to simulate half-space steady evaporation or condensation in Direct Simulation Monte Carlo simulations are adapted to Molecular Dynamics (MD). A microscopic definition of evaporation and condensation is introduced and values for the evaporation and condensation coefficients are calculated from MD. The velocity distribution functions for the evaporation and condensation modes have been calculated and compared with the standard assumptions in gas-kinetic calculations.