The General Data Protection Regulation grants data subjects the legal rights of transparency and intervenability. Ex post transparency provides users of data services with insight into how their personal data have been processed, and potentially clarifies what consequences will or may arise due to the processing of their data. Technological artefacts, ex post transparency-enhancing tools (TETs) convey such information to data subjects, provided the TETs are designed to suit the predisposition of their audience. Despite being a prerequisite for transparency, however, many of the TETs available to date lack usability in that their capabilities do not reflect the needs of their final users.

The objective of this thesis is therefore to systematically apply the concept of human-centred design to ascertain design principles that demonstrably lead to the implementation of a TET that facilitates ex post transparency and supports intervenability. To this end, we classify the state of the art of usable ex post TETs published in the literature and discuss the gaps therein. Contextualising our findings in the domain of fitness tracking, we investigate to what extent individualisation can help accommodate the needs of users of online mobile health services. We introduce the notion of privacy notifications as a means to inform data subjects about incidences worthy of their attention and examine how far privacy personas reflect the preferences of distinctive groups of recipients. We suggest a catalogue of design guidelines that can serve as a basis for specifying context-sensitive requirements for the implementation of a TET that leverages privacy notifications to facilitate ex post transparency, and which also serve as criteria for the evaluation of a future prototype.