The Arctic research base AWIPEV is located at the western coast of Spitsbergen, on the rim of the Arctic Ocean. It is in operation year-round. The mild climate here is due to the warm West-Spitsbergen current and is in contrast to the harsh climate on Greenland or the ice covered ocean of the same latitude. The dedicated Atmospheric Observatory collects continuously meteorological parameters, and incoming and outgoing radiation at the surface. Vertical profiles of pressure, temperature, humidity, and wind are measured by balloon-borne sondes on a daily basis up to stratospheric heights of 30 km. This includes weekly ozone measurements (see Rex et al. this issue). Remote sensing techniques are employed to observe aerosols and trace gases using spectrometers, microwave-radiometers, and lidar. This paper presents the main findings of 20 years of observations.