This chapter describes the functional neuroanatomical components of olfaction, and summarises the findings in various neurological and psychiatric disorders. The functional significance of the components of the olfactory cortex have been determined from experiments in animals, human lesion studies neurological disorders and, more recently from work using newer brain imaging techniques, such as positron emission tomography (PET), and both structural and functional magnetic resonance imaging (fMRI). Independence of function of olfactory abilities has been suggested by studies in neurological patients with lesions in various parts of the olfactory system. Deficits in olfactory identification are often associated with lack of awareness of olfactory dysfunction, and are related to injury severity. Examination of olfactory disturbances may provide early markers of impending neurological or psychiatric illness and, in some disorders including Alzheimer's Disease (AD), attention deficit hyperactivity disorder (ADHD) and schizophrenia, may be trait markers of the condition.