Subjects participating in the RNS® System Feasibility study are required to have successfully completed the non-significant risk Prospective Seizure Frequency (PSF) study, which gathers baseline(pre-implant) seizure frequency data. Subjects must also met the inclusion criteria, including localization of epileptogenic region(s), prior to enrolling in the study. Throughout the entire study, subjects or their caregivers must keep a seizure diary. Seizure frequency, seizure severity, and antiepileptic medications, as well as physical and emotional health will be monitored and recorded throughout the study. Antiepileptic medications should continue to remain stable until 5 months post-implant.