Transient epileptic amnesia (TEA) is a form of epilepsy of which the principle manifestation is recurrent, transient episodes of isolated memory loss. Although the phenomenon has been recognised for over a century, it is scantily documented in the medical literature and is often misdiagnosed by clinicians. Recent work has highlighted a number of apparently consistent clinical features among the published cases. However, to date there has been no large, systematic study of the condition. The aim of the work reported in this thesis was to investigate a substantial number of prospectively recruited patients with TEA, and thus be able to provide a detailed and authoritative description of its clinical, neuropsychological and radiological characteristics. Fifty patients with TEA were recruited from around the United Kingdom using established diagnostic criteria, together with a group of matched healthy control subjects. Participants underwent a clinical interview, comprehensive neuropsychological testing and structural magnetic resonance imaging of the brain. The study demonstrated the following features. TEA typically begins in later life. The amnesic episodes are frequent, brief and often occur upon waking. They are characterised by a mixed anterograde and retrograde amnesia, the anterograde component of which is often incomplete. Attacks are commonly associated with olfactory hallucinations. They respond well to anticonvulsant medication. Nevertheless, many patients complain of persistent difficulties with memory. Despite generally performing well on standard tests of anterograde memory, many patients show i) accelerated forgetting of new information over a three-week delay and ii) temporally extensive deficits in autobiographical memory. TEA is associated with subtle medial temporal lobe atrophy on magnetic resonance imaging. This atrophy correlates with performance on standard memory tests, but not with long-term forgetting rates or autobiographical memory deficits. It is proposed that TEA is a distinctive syndrome of epilepsy, typically misdiagnosed at presentation, caused by medial temporal seizure activity and associated with accelerated long-term forgetting and autobiographical memory loss. These unusual forms of memory impairment have been documented in other forms of epilepsy. They pose challenges to current models of memory. The syndrome of TEA is therefore both clinically and theoretically important.