This study will evaluate the long term safety, tolerability, pharmacokinetics (PK), and efficacy of XEN1101 in subjects with Focal Onset Seizures (FOS) or Primary Generalized Tonic-Clonic Seizures (PGTCS) for the treatment of seizures for up to 3 years.

To demonstrate that the RNS System is safe and effective as an adjunctive therapy in individuals age 12 through 17 years with medically refractory partial onset epilepsy.