Inclusion Criteria:  Have agreed to participate in the observatory after receiving written information on the purpose of the study and the personal data to be collected (agreement of parents for under-age patients); Present with haemophilia B and are currently being treated or have previously been treated with Idelvion® as a long-term prophylactic treatment, on-demand treatment or short-term treatment for surgical procedures or to cover periods with a high-risk of bleeding. Do not have FIX targeted antibodies at the time of the inclusion visit  Exclusion Criteria:  Refusal by the patient or his/her legal representative to participate in the study; Existence of a contraindication to the use of Idelvion® treatment (known hypersensitivity to FIX or hamster proteins); Simultaneous participation in an interventional clinical study