Inclusion Criteria:  Aged ≥4 years and <18 years. Enrolled in the XLH Registry via one of the participating UK centres.  Have at least one of the following:  RSS calculated at enrolment during a routine clinic visit. OR Historical radiographs of the affected wrist and/or knee available in the medical chart within the 6 months prior to enrolment, to allow RSS to be centrally calculated retrospectively. Receiving any XLH treatment (burosumab, oral phosphate and active vitamin D, or other XLH treatment) within 30 days prior to informed consent for this study  Exclusion Criteria:  Does not have the cognitive capacity to provide informed consent, or their legally designated representative (i.e. parent / legal guardian) does not have the cognitive capacity to provide informed consent. Is not expected to have open growth plates for the duration of the study. Currently participating in an interventional clinical trial. Participation in a Compassionate Use Programme, Pre-commercial Programme, or Investigator Initiated Study does not preclude a patient from participation in this study.