The goal of this diploma thesis was to specify and implement an XML-based Billing Agent - fulfilling the de-facto standard specifications of IPDR.org - for the CIPnG-based Public Wireless LAN that had been implemented by former ZHW students for futureLAB Ltd. in Winterthur, Switzerland. The Public Wireless LAN needs a Billing Infrastructure to be able to track down the use of resources by the customers on the one hand and the distribution of the traffic to the different base stations on the other hand. So, the network operator is able to produce billings for his customers and pay reimbursements to the operators of the Base Stations. According to the posting of this thesis, the billing data should have been easily extracted out of the CIPnG-Gateway?s Linux Netfilter Module, from where a Java Based XML Agent should parse the data, produce Billing Data Output in accordance to IPDR.org and display it in a convenient way. Unfortunately, the CIPnG-Gateway?s Linux Netfilter Module caused overwhelming troubles for us: the needed data was not present in it, nor was it to be extracted easily. More than 50% of the resources had to be put in that issue. For the moment, just parts of the new designed Netfilter Modules do really work. Nevertheless, we could specify and implement such a Billing System that produces IPDR XML output, using Java-, JDBC-, PostgreSQL- and XSLT- technologies. In addition, we drafted new IPDR schemas for the existing ones that never had been used and did not work. We also specified some new, proprietary XML schemas for displaying Billing Data for those use cases that could not be satisfied by the IPDR specifications.