Effectiveness Study Comparing Treatment With Drug(s) or Adjunctive VNS Therapy for Pharmacoresistant Partial Seizures

Inclusion Criteria: * Patient has a diagnosis of partial seizures with onset before age 50, and the patient's prior electroencephalography and magnetic resonance imaging studies are consistent with the diagnosis of partial seizures. * Patient has at least 4 complex partial seizures, but less than 25 seizures (any type), per month during the 3 months preceding randomization; for the 8 weeks preceding randomization, the seizure frequency should be confirmed from a patient diary. * Patient has not had an adequate response to an adequate dosage of -- or was intolerant to -- a minimum of 2 different AEDs. * Patient has (in the investigator's judgment) sufficient impairment from his/her epilepsy and/or epilepsy treatment that the potential benefits/risks of VNS therapy are warranted. * Patient must currently be receiving at least one AED, but not more than three AEDs, in a stable dosage regimen for at least one month before randomization. * Patient must be 12 years of age or older. * Patient must be able to provide reliable seizure counts and to complete the evaluations specified in the study procedures flow chart. * Patient must provide written informed consent, or legal guardian must give written permission and the minor provide written assent. Exclusion Criteria: * Patient has a history (lifetime) of having received more than 5 different AEDs. * Patient has had a bilateral or left cervical vagotomy. * Patient currently uses, or is expected to use during the study, short-wave diathermy, microwave diathermy, or therapeutic ultrasound diathermy. * A VNS Therapy System implant would (in the investigator's judgment) pose an unacceptable surgical or medical risk for the patient. * Patient is expected to require full body magnetic resonance imaging during the clinical study. * Patient has had a previous VNS Therapy System implant. * Patient has a previous neuroimaging study that demonstrates mesial temporal sclerosis, cortical dysplasia, or a suspected brain tumor.