Objective: Generate prospective follow-up data to increase our understanding of this rare disease. Study design: Prospective observational cohort study spanning 5 years, during which we will collect standard clinical data as well as blood samples and quality of life questionnaires. Study population: hepatitis delta patients aged ≥18 years Intervention (if applicable): not applicable Main study parameters/endpoints: Incidence of liver related events (liver cancer, (decompensation of) cirrhosis, liver transplantation) during follow-up and changes in markers of viral replication, inflammatory activity and liver stiffness over time. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The risks associated with participation can be considered negligible and the burden can be considered minimal. The only additional action that the participants must perform are the filling out of two annual quality of life questionnaires, which are considered non-invasive, and collection of 10 ml blood during regular blood sample collections