Space borne observations by a high performance polar orbiting lidar, initiated in 2003 by the launch of the Geoscience Laser Altimeter System (GLAS), have provided fundamentally new results on the global distribution of clouds and aerosol. The active detection method of lidar gives unambiguous retrieval of the height and vertical structure of cloud layers and has provided the first global measurements of boundary layer height. The applications of the GLAS measurements include validation of cloud, aerosol transport and boundary layer models, cloud and aerosol climatology studies, cloud/aerosol interaction, polar stratospheric clouds, dust and smoke transport and many others. The initial analyzed results for GLAS show the total global cloud cover is 70% and cloud overlap, defined as the detection of a second cloud layer before full optical attenuation, is found to be present in 40% of cloudy areas globally. For atmospheric aerosol, regions of heavy loading with elevated aerosol layers above and overlapping clouds are a normal occurrence. In comparison to passive satellite retrievals, large improvements are provided in some crucial cloud and aerosol parameters. From interpretation of the aerosol height distribution a unique global data product of PBL height has been produced. The PBL height data, and cloud distribution, have been applied as a new validation for the ECMWF and other global circulation models. A unique feature of the GLAS data is a precision measurement of the return pulse shape and magnitude from the surface as a calibration and boundary condition for atmospheric retrievals. Even though the instrument reliability has not been as good as planned, GLAS has obtained about 12 months of good quality data is available and intermittent operation continue. In this talk we give an overview of the GLAS mission and show results thus far obtained in these and other research areas.