This chapter reviews the literature on olfactory hallucinations and discerns any characteristics of olfactory hallucinations which allow those of organic aetiology to be distinguished from those of functional origin, such as those related to schizophrenia. Olfactory hallucinations/auras are rare in patients with epilepsy but may be more common when the epilepsy type is restricted to temporal lobe epilepsy (TLE). The prevalence of olfactory hallucinations in psychiatric populations has varied widely. Olfactory hallucinations have been described in patients with differing diagnoses including schizophrenia, affective disorders, eating disorders and hysteria. Olfactory hallucinations are observed in patients with schizophrenia but usually in combination with the other symptoms of schizophrenia and with hallucinations in other modalities. There are no clear-cut qualitative differences between the olfactory hallucinations described by patients with schizophrenia compared to patients with 'organic states', though the presence of continuous olfactory hallucinations is more suggestive of a psychiatric diagnosis.