The General Data Protection Regulation (GDPR) specifies a series of transparency obligations for data controllers concerning the processing and sharing of personal data. Transparency architectures focus on securely and efficiently logging private data events to enable either data subjects or trusted auditors to verify that the processing of personal data has happened In accordance with agreed privacy policies and the existing regulations. This project investigates the advantages and limitations of different candidate architectures for transparency logging. We evaluate existing solutions in terms of features such as Confidentiality, Integrity, Immutability, Interoperability, and Traceability. Furthermore, the project uses the results of the gap analysis to identify the primary open challenges and opportunities for future research.