This is a multicentre observational study with prospective and retrospective data collection and retrospective data collection and biological collection from patients with HBV/HDV co-infection.

This is an observatory for patients co-infected with hepatitis B and Delta viruses. Patients will be monitored according to the usual recommendations, depending on their status:

• Patients who have never received specific treatment for hepatitis Delta (untreated or receiving treatment with peginterferon alpha 2a alone) will be monitored according to current recommendations, once every 6 months;
• Patients treated or having been treated with a specific hepatitis Delta treatment will be monitored according to the compassionate access protocol or according to the recommendations of the AMM during treatment and according to routine follow-up after the end of treatment.

Participation in research entails the following additional procedures for patients, for each line of treatment, where applicable:

• Samples for the biobank,
• Self-administered questionnaires.

In addition, as sub-studies are planned on sub-groups of patients, these sub-studies may involve additional constraints/interventions

Inclusion Criteria:

• Age > 18 years,
• Presenting a chronic HDV infection (positive serology),
• Who gave his written informed consent before any intervention and the day of inclusion at the latest,
• Affiliated to Health Insurance or to the "Aide Médicale d'Etat" (request for exemption pending).

Exclusion Criteria:

• Patient participating in another biomedical research with an exclusion period ongoing at inclusion,
• Vulnerable patient (minor, adults legally protected: under judicial protection, guardianship, or supervision, persons deprived of their liberty).
• Patients with predictable difficulties of follow-up according to the investigator.

All patients followed in the participating centers for their HBV/HDV co-infection are susceptible to be included.

Patients who already started a specific treatment for their HDV infection will be included retrospectively, after they have signed an informed consent.