Reactivation of hepatitis B virus (HBV) is a known complication of immune-suppressive, cytotoxic, and biological modifier therapies in patients currently infected with HBV or who have had past exposure to HBV. Nowadays, newer and emerging forms of targeted biologic therapies are available for the management of rheumatologic conditions, malignancies, inflammatory bowel disease, dermatologic conditions and solid-organ, bone marrow, or haematologic stem cell transplant but there is currently a lack of a systematic approach to the care of patients with or at risk of HBV reactivation. The Hepatology Committee of the European Society of Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) together with a working group of ESPGHAN members with clinical and research expertise in viral hepatitis developed an evidence-based position paper on reactivation of HBV infection in children identifying pertinent issues addressing the diagnosis, prevention, and treatment of this condition. Relevant clinical questions were formulated and agreed upon by all the members of the working group. Questions were answered and positions were based on evidence resulting from a systematic literature search on PubMed and Embase from their inception to July 1, 2019. A document was produced and the working group and ESPGHAN Hepatology Committee members voted on each recommendation, using a formal voting technique. A recommendation was accepted provided upon agreement by at least 75% of the working group members. This position paper provides a comprehensive update on the diagnosis, prevention and treatment of HBV reactivation in children.