Controllers may define the context in which pseudonymisation is to preclude attribution of data to specific data subjects. This context will be called the pseudonymisation domain in these guidelines. The pseudonymisation domain does not have to be all-encompassing, but may be restricted to defined entities, most often to the set of all authorised recipients of the personal data that will process the data for a given purpose. The effectiveness of pseudonymisation in the implementation of data- protection principles or in the assurance of a level of security appropriate to the risk is highly dependent on the choice of the pseudonymisation domain and its isolation from additional information that allows the attribution of pseudonymised data to specific individuals. Thus, pseudonymisation is a safeguard that can be applied by controllers to meet the requirements of data protection law and, in particular, to demonstrate compliance with the data protection principles in accordance with Art 5(2) GDPR. These guidelines will help controllers to choose effective techniques for the modification of original data, to protect pseudonymised data from unauthorised attribution, and to manage user rights when processing pseudonymised data. Controllers must always bear in mind that pseudonymised data, which could be attributed to a natural person by the use of additional information, remains information related to an identifiable natural person, and thus is personal data (Rec. 26 GDPR). Therefore, the processing of such data needs to comply with the GDPR, including the principles of lawfulness, transparency, and confidentiality under Art. 5 GDPR, and the requirements of Art. 6 GDPR. Controllers must maintain an appropriate level of security by implementing further technical and organisational measures. Finally, controllers must ensure transparency, and need to facilitate the exercise of the data subject rights set out in Chapter III of the GDPR, unless the exception provided for in Art. 11(2) and 12(2) GDPR applies. Adopted - version for public consultation