Epileptic seizures are often un-witnessed and can result in hypoxic brain damage or can be fatal due to injuries, status epilepticus or sudden unexpected death in epilepsy (SUDEP). The first aim of this thesis was to investigate some of the physiological parameters that accompany an epileptic seizure and may be useful in a seizure alarm system. The second aim was to investigate aspects of cardiac dysfunction during clinical and sub-clinical seizures that may be potential contributing factors in SUDEP. Percentage heart rate change and oxygen saturation were studied prospectively during 527 epileptic seizures in 50 patients aged from one-day full term neonate to 60 years with a variety of seizure types (absences, generalised tonic clonic seizures, myoclonic seizures, tonic seizures and focal seizures) and in normal physiological events (e.g. coughing, turning in bed). Higher percentage heart rate change occurred during epileptic seizures (21.8%) than during normal physiological events (16.4%) p<0.001. Diagnostic testing of clinically significant seizures i.e seizures that could potentially lead to serious consequences if left undetected (n=61) had a sensitivity of 91% and specificity of 75% when percentage heart rate change and hypoxaemia parameters were combined. Percentage heart rate change and oxygen saturation could be used as reliable indicators of a seizure when set at specific levels and distinguish clinically significant seizures from normal physiological events. These parameters can now be used to develop a reliable alarm system to detect epileptic seizures at night. Prolongation of QTc and increased vagal tone may be possible mechanisms underlying SUDEP. Corrected Q-T cardiac repolarisation time 5 minutes before and throughout 156 epileptic seizures were analysed using four corrective formulae (Bazett, Hodge, Fridericia and Framingham). All formulae indicated statistically significant lengthening of the corrected Q-T during epileptic seizures (p<0.001) compared to pre-seizure values. All formulae agreed that the greatest lengthening of the corrected QT beyond normal limits occurred during right temporal lobe seizures in two patients. Reflex and tonic vagal activity utilising R-R intervals was assessed in 33 sub-clinical seizures occurring during stages 3 or 4 sleep and was compared to matched counts of R-R interval non-ictal baseline studies from the same stage of sleep in each patient. Altered vagal activity occurred during total sub-clinical seizures compared to baseline studies (p<0.001). Lengthening of the corrected Q-T and changes in cardiac vagal tone during epileptic seizures may have a role in the patho-physiology of SUDEP.