In an ever increasing dynamic environment, companies are forced to cooperate in order to meet customer needs efectively. They set up Service Networks (SN) trying to create a win-win situation for all participants of the network. The calculation of value in an SN is based on key performance indicators (KPIs) which measure the performance of underlying cross-organizational business processes. As for the calculation of KPIs of these processes monitoring information from several paricipants is needed, in an SN it is no more suficient for the participants to monitor just KPIs of their internal processes, e.g., by using Business Activity Monitoring technology. The participants now have to provide a set of monitoring events to the other partners in the SN. In this paper, we describe an approach to monitoring of KPIs across partners in a service network. An SN is mapped to a service choreography and a monitoring agreement is created which specifes how KPIs are decomposed to events that participants in the choreography have to provide. We present our approach based on a case study from the telecommunications domain.