Inclusion Criteria:

• * Any age or sex with a confirmed diagnosis of pediatric-onset HPP (that is, first HPP sign or symptom presented at \< 18 years of age).
• * Currently receiving asfotase alfa treatment at Enrollment (not treatment-naïve) or the Physician has decided to resume (not treatment-naïve) or start (treatment-naïve) the participant's asfotase alfa treatment within 6 months after Enrollment.
• * Participant must have documented alkaline phosphatase (ALP) activity below the lower limit of normal for age and sex, and a documented ALPL gene mutation (Note: An exception is made for infants with clinical features of HPP plus low ALP who need to start asfotase alfa treatment right away, at the Physician's discretion, but do not yet have a genetic result. In this case, ALPL gene documentation is not required at the time of sub-study enrollment but should be documented within 6 months after Enrollment).
• * Participant or participant's parent/legally authorized representative is able to read and/or understand the informed consent and study questionnaires in the local language.
• * Participant or participant's parent/legally authorized representative must be willing and able to give signed informed consent for this sub-study, and the participant must be willing to give written informed assent, if appropriate and required by local regulations.

Exclusion Criteria:

• * Currently participating in an Alexion-sponsored interventional clinical study. Participants who have concluded participation in an Alexion-sponsored asfotase alfa clinical study are eligible to enroll in this sub-study.