Inclusion Criteria:  Drug-resistant epilepsy indicated to VNS based on clinical decision. Drug-resistant epilepsy is defined as a failure of adequate trials of two tolerated, appropriately chosen, and used antiepileptic drug schedules (whether as monotherapy or in combination) to achieve sustained seizure freedom (Kwan et al., 2010). Age ≥ 18 years Availability to record 20 minutes of EEG with photic stimulation, hyperventilation, and eye-opening/closing according to the pre-defined protocol. Availability of seizure diaries at least three months before VNS implantation OR reliable information about seizure frequency at least three months before VNS implantation The ability of a patient/family member/caregiver to record seizures precisely into seizure diaries OR the ability of a patient/family member/caregiver to report seizures precisely different ways. In cases with very high seizure frequency (several seizures per day), it is acceptable to report only the days without any seizures.  Exclusion Criteria:  The indication and planning of resection brain surgery as a treatment option for drug-resistant epilepsy. If a patient clearly demonstrates his refusal of resection surgery, he/she can be included in the study. The presence of psychogenic non-epileptic seizures which cannot be reliably distinguished from epileptic seizures by a patient/family member/caregiver. The presence of other condition which can resemble epileptic seizures which cannot be reliably distinguished from epileptic seizures by a patient/family member/caregiver. Metabolic condition or other diseases, in which the increase of seizure frequency is expectable. The inability of a patient to take regular visits which are required for VNS parameters settings or the study Life expectancy shorter than two years