The atmospheric boundary layers with a wide range of stability are simulated experimentally using a thermally stratified wind tunnel and numerically by DNS. The turbulence structure and flow characteristics of stratified boundary layers under both stable and unstable conditions are investigated For stable boundary layers (SBL), attention is focused on the buoyancy effects on stratified turbulent flows with strong stability. Wave-like motions driven by buoyancy and waves due to the Kelvin-Helmholtz instability can be observed in the lower part of boundary layers with very strong stability. Simulations on the convective boundary layers (CBL), capped by a strong temperature inversion and affected by surface shear, are also carried out. The comparison of the wind tunnel data and DNS results with those of atmospheric and water tank studies of CBL shows the crucial dependence of the turbulence statistics in the upper part of the layer on the strength of inversion layer, as well as the modification of the CBL turbulence regime by the surface shear.