One of the primary aims of the GDPR is to increase the participation of data subjects and to grant them more control over their data. Since Google and Facebook are global companies collecting and processing vast amounts of personal data, their compliance with the GDPR is essential to ensure better protection of the data subjects. The key challenge now is whether the GDPR impacts the business model of Facebook and Google, since the scandal of Cambridge Analytica revealed that "transparency" has no meaning in today's online market, where individuals' data is the currency. Even though GDPR provides some fundamental rights for the data subjects, it could be argued that the data subjects have no effective control over their data in the online market in practice. Therefore, it is necessary to analyse, whether GDPR sets adequate safeguards regarding the data subjects' rights, regarding data owners' consent, purpose limitation principle and the right to be informed as well as whether these companies meet the requirements of the GDPR in practice. Thus, the aim of this book is to illuminate the interaction of legislative decree and business practices of Facebook and Google.