In the fission process of a nuclear explosion a large amount of radioactive Xenon is produced. Since Xenon is a noble gas which, in contrast to radioactive particles, is likely to leak out even from a subsurface nuclear explosion. Therefore, radioactive Xenon and possibly other noble gases, are in fact the only elements that if detected can unmistakably verify an event as a nuclear explosion. The Swedish Automated Unit for Noble gas Acquisition (SAUNA) was developed at FOI. It automatically collects and processes 12 hour samples of ambient air and measures the radioxenon concentration in the sample. The system purifies each sample and extracts the Xenon content and finally measures the activity of the sample using a sensitive beta-gamma coincidence detector. During the last six years more than 20 SAUNA systems has been produced. The main usage is in the global network run by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in order to verify the treaty that was signed in 1996. This talk will present how the system works and how the measured data can be used to verify if a nuclear explosion has occurred.