The Dutch implementation of the Data Retention Directive was adopted on the 1st of September, 2009. The main reason for the storage of call detail records of telephone and internet traffic data is its potential in the aid of the investigation and prosecution of serious crimes. However the fact that this data has to be stored for a certain period of time is a recurring point of debate. There is a need both in the Netherlands and at European level for a clearer understanding of how the police and judicial authorities use the data kept under the Telecommunications Data Retention Act. The purpose of this study is to clarify how the Act works in practice.