The RNS® System LTT study will provide additional data on the safety and efficacy of the RNS® System for 7 years following a subject's completion of the RNS® System Feasibility or Pivotal studies. Data from the RNS® System LTT study will be combined with data collected during the RNS® System Feasibility and Pivotal studies, resulting in 9 total years of post-implant follow-up data. These data will be used to calculate long-term SAE rate, percent change in seizure frequency (from pre-implant baseline), as well as the frequency of sudden unexplained death in epilepsy (SUDEP).