A new Europe-wide study has been launched to into the causes of ozone depletion over the Arctic and mid-latitude regions. The Second European Stratospheric Arctic and Mid-latitude Experiment (SESAME) is a direct response to the dangerous depletion of the ozone layer over northern Europe, which has accelerated beyond the predictions of current models over the past few years. SESAME runs from January 1994 to May 1995 and will include laboratory measurements, theoretical studies, field measurements and an extended field campaign. Over 50 groups from the European Economic Area (Twelve EU and six EFTA states) will be involved, in collaboration with many non-European colleagues. Over 300 scientists will take part. Most of the field activity for SESAME will be concentrated in three intensive phases between January and March 1994, September and October 1994, and December 1994 to April 1995. The programme will take measurements from a network of 30 ground-based sites and 26 ozone-probe launching sites. Large stratospheric balloons will be launched from two locations in Northern Scandinavia and Southern France to directly measure changes in the ozone layer. In addition, four aircraft will take measurements in the polar regions and over the populated middle latitudes. These coordinated measurements and the associated programme of analysis are expected to make a major contribution to the study of global ozone loss. Such studies are required in order that policy decisions on environmental regulation are based on the soundest scientific advice. The European Community is funding SESAME through the Community Environment Research Programme as a contribution to the on-going World Meteorological Organisation/United Nations Environment Programme assessment of the ozone layer, required by the Montreal Protocol on substances that deplete the ozone layer.