This is a 10-year multi-center, global, observational study to further characterize the safety profile of pegvaliase, including hypersensitivity reactions, long-term safety and tolerability, and the effectiveness of the additional risk minimization measures (aRMMs) (European Union (EU) only) in subjects receiving pegvaliase for the treatment of PKU. Subjects for whom a clinical decision has been made that they will receive pegvaliase to treat their PKU within 30 days following the date of enrollment (incident-users) or have previously started treatment with pegvaliase at the date of enrollment (prevalent-users) are eligible for participation in this study.