Since a few years, we are using crowdsourcing as a significant addition to classical methods for collecting network performance data and measuring quality of experience from the end user’s point of view. Our app-based, on-device data collection engine is working passively in the background of a user’s smartphone. With this app-based approach, end-user smartphones become measurement devices delivering invaluable knowledge on how customers use and experience mobile communications. The data collection engine is integrated into more than 800 applications worldwide and generates more than 3,5 billion data samples per day. Basis for integrating the data collection engine is a cooperation between us and the application providers. While installing one of their applications the user is normally informed about the additional software by the usage terms and conditions. He has the choice to accept it or not.