Autonomic function may help to localize and manage the epilepsies. It is likely that the mechanisms of Sudden Unexpected death in Epilepsy (SUDEP) involve autonomic disturbance and a better understanding of these might lead to measures that would help reduce the mortality in patients afflicted with epilepsy. In this thesis, I first provide a comprehensive literature review of the association between epilepsy and the autonomic nervous system. I then evaluate heart rate variability and other cardiac and endocrine parameters as indices of cardiac autonomic function to test three hypothesis; 1) Changes in heart rate variability (HRV), can occur in the peri-ictal period during both (a) subclinical electrographic seizures and (b) clinically overt partial seizures, and can help to localise and lateralise the ictal discharge. 2) Intractable epilepsy can disrupt the heart rate variability and its circadian rhythm. 3) Epileptic seizures affect the serum concentration of the catecholamines and the electrolytes and that these changes could impact on the corrected QT interval. Subjects (n=207) with intractable epilepsy who were being evaluated with video-EEG telemetry for epilepsy surgery were recruited for this study. I found that subclinical seizures have no effect on the HRV. However, in overt partial seizures, HRV decreases, corrected QT is prolonged and plasma catecholamines increases. The reduction in HRV during seizures is not affected by the hemispheric or lobar location of the epileptic focus. However, in the interictal period, reduced HRV differs in left vs. right hemisphere, and in temporal vs. extratemporal areas. The diurnal pattern of HRV is not altered in epilepsy and the mean day HRV were significantly different from mean night HRV. The reduction in HRV is also associated with the following clinical factors: prolonged medical history of epilepsy, the cortical pathology itself, the nature of the seizures, higher seizure frequency and the antiepileptic drug treatment. The plasma electrolytes: Na, K+, Ca2+ and cardiac troponin are not affected after a seizure. However, plasma Mg2+ was seen to increase after a seizure. These abnormalities in autonomic control, particularly the reduction in HRV might be one contributory mechanism of Sudden Unexpected Death in Epilepsy (SUDEP).