An overview is provided of the COST 723 Action, Data Exploitation and Modelling of the Upper Troposphere and Lower Stratosphere. The three working groups are introduced and a summary of Action activities within them is provided. The achievements of the Action are: three international workshops; the LAUTLOS humidity measurement campaign; dedicated meetings to discuss the quality of upper troposphere/lower stratosphere ozone and humidity measurements; two journal special issues; more than 90 papers in the peer-reviewed literature; one international summer school; and a successor COST Action which builds on COST 723. The recommendations made are: for COST to continue to support the short-term scientific missions instrument, as they are perceived to be value for money; to encourage the use of COST money to increase links between COST Actions and other scientific communities; and for the COST secretariat to recommend that Actions consider a summer school instead of a final workshop or meeting. ; An overview is provided of the COST 723 Action, Data Exploitation and Modelling of the Upper Troposphere and Lower Stratosphere. The three working groups are introduced and a summary of Action activities within them is provided. The achievements of the Action are: three international workshops; the LAUTLOS humidity measurement campaign; dedicated meetings to discuss the quality of upper troposphere/lower stratosphere ozone and humidity measurements; two journal special issues; more than 90 papers in the peer-reviewed literature; one international summer school; and a successor COST Action which builds on COST 723. The recommendations made are: for COST to continue to support the short-term scientific missions instrument, as they are perceived to be value for money; to encourage the use of COST money to increase links between COST Actions and other scientific communities; and for the COST secretariat to recommend that Actions consider a summer school instead of a final workshop or meeting.