This is a multicenter, prospective, controlled study designed to evaluate treatment with the BTS System.

Inclusion Criteria:

1. Subject is greater than or equal to 7 years of age.
2. Subject has focal onset seizures.
3. Subject has failed treatment with a minimum of two AED's used in typical therapeutic dosages.
4. Subject has seizures that are distinct, stereotypical events that can be reliably counted by the patient or caregiver.
5. Subject can reasonably be expected to maintain a seizure diary and the BTS System alone, or with the assistance of a competent individual.
6. For one month prior to enrollment, subject's anti-epileptic medication dosages and ketogenic diet (as applicable) have been stable (other than acute, intermittent use of benzodiazepines) and subject has had at least three primary, disabling seizures per month, on average. Seizures must be separated by a minimum of eight hours not to be considered part of a cluster. A cluster, for the purpose of this criterion, shall be considered a single seizure.
7. Subject is able to complete regular office visits and telephone appointments in accordance with the study protocol requirements.
8. A female subject of childbearing age must have a negative serum pregnancy test within two weeks prior to implant of the INSR, and, if sexually active, must be using a reliable form of birth control, be surgically sterile, or be at least two years post-menopausal.
9. Subject has been informed of their eligibility for resective surgery as a potential alternative to the study, if such surgery is a reasonable option.
10. Subject has had a brain MRI epilepsy evaluation within the past two years.
11. Subject's anatomy will permit implantation of the INSR within 20 mm of the skin surface.

Exclusion Criteria:

1. Subject has a history of substance abuse within the preceding two years.
2. Subject participated in another drug or device trial that may confound study results within the preceding 30 days.
3. Subject is implanted with pacemaker, implantable cardiac defibrillator, cardiac management product, or a medical device that interferes with the BTS System or with which the BTS System interferes. Patients with a vagus nerve stimulator (VNS) implanted may be enrolled, provided their clinical status has been stable for at least one month prior to enrollment at their current stimulation parameter settings.
4. Subject has anatomy that may interfere with electrode placement.
5. Subject is on anticoagulants and is unable to discontinue them perisurgically, as required by the neurosurgeon or Investigator.
6. Subject has significant platelet dysfunction from medical conditions or medications (including, particularly, aspirin or sodium valproate). If platelet dysfunction is suspected, subject can be enrolled only if a hematologist, the Investigator, and the neurosurgeon judge it to be advisable.
7. Subject has been diagnosed with psychogenic or non-epileptic seizures that cannot be distinguished from their epileptogenic events.
8. Subject is ineligible for cranial surgery.