Inclusion Criteria:  Confirmed diagnosis of XLH (documented diagnosis of XLH in medical records, and evidence of at least one of the following: hypophosphataemia and/or impaired phosphate reabsorption due to elevated FGF23; PHEX mutation). Aged 12 to 17 years at start of study. Has open growth plates at enrolment and is estimated by their treating clinician to reach end of skeletal growth within the next 26 weeks (based on clinician's judgement in accordance with their normal approach used in routine practice). Has been receiving treatment with burosumab for at least study le (52 weeks). Provides informed consent to take part in the study (or provides assent, and carer provides consent, where applicable in accordance with specific country regulations).  Carer Inclusion Criteria:  A main carer of a study participant (i.e. a parent or guardian who provides day-today support or care for the adolescent with XLH who is taking part in this study). Provides informed consent to take part in the study (for self and/or on behalf of eligible adolescent, where applicable in accordance with specific country regulations).  Exclusion Criteria:  Unwilling and unable to participate in all aspects of the study (i.e. interviews, app, EQ- 5D-Y, wearable data collection) and /or does not agree to the collection of data from medical records. Missed two or more injections of burosumab in the past 6 months. Is planned to have any surgery during the study period.