The diagnosis of epilepsy carries significant implications for physical, psychosocial and financial wellbeing as well as a small but significant increased risk of mortality. The diagnosis is often incorrect, potentially in up to 20% of cases, so should be revisited if seizures are not responding to treatment. Evidence indicates that misdiagnosis is significantly more common among nonspecialists. SIGN recommends that the diagnosis of epilepsy should be made by an epilepsy specialist, ideally in the setting of a dedicated first seizure or epilepsy clinic. An incorrect diagnosis of epilepsy can be harmful. There is an exhaustive list of epilepsy mimics that can result in misdiagnosis and expose patients to unnecessary treatment with antiepileptic drugs. Diagnosis relies primarily on the history. Investigations can support the diagnosis but cannot make it in isolation, and negative investigation findings are common in epilepsy. Brain imaging will be undertaken in most patients with epilepsy, but is not routinely required in those with a definite diagnosis of genetic generalised epilepsy. The EEG has limitations and can sometimes cloud rather than clarify the diagnostic picture. Distinguishing between a genetic generalised epilepsy and a focal epilepsy is vital as this influences investigation, treatment and prognosis. Generally medication should not be started following a single seizure except in specific circumstances or in cases where the risk of recurrence is high.