Grazing incidence X-ray reflectivity (GIXR) is a powerful technique for investigating surfaces and multilayers. Three different material systems, GaAs/AlAs, SiO2/Si, and Si3N4/Si multilayers, were studied by GIXR. Structural parameters such as thickness, density and roughness of these multilayers were successfully determined with high accuracy. Sub-nm resolution was achieved for both thickness and roughness measurements. In addition, GIXR was found to be more sensitive to surface roughness than underlayer interface roughnesses.