Each time users connect to the internet or make a phone call, they leave ‘digital fingerprints’ which may turn out to be useful for identifying people involved in a crime or an act of terrorism. This is why there is great interest by law enforcement authorities in gaining access to traffic and location data, relying on the forced co-operation of telecommunication and internet providers. However, the effectiveness of compelling these service providers to retain such data for the purpose of fighting crime has been questioned. Moreover, data retention obligations interfere with the right to privacy of individuals, as protected by Article 8 European Convention of Human Rights (ECHR); therefore, they may be imposed only for a legitimate objective, i.e. to protect citizens' safety, and in full respect of the principle of proportionality.