This chapter discusses CSF rhinorrhoea, which is the flow of cerebrospinal fluid from the nose which can occur spontaneously or because of varied aetiological factors. Usually, the sites of CSF leak are cribriform plate, frontoethmoid and sphenoid sinuses which have an intimate relationship with the nasal cavity. CSF is formed by choroid plexus of ventricles and may be by capillaries on the surface of brain and fills the subarachnoid space. Total volume is 140 ml and rate of production of CSF is 0. 2-0. 4 ml/minute. Main constituents of CSF are water, sodium chloride, glucose, proteins and cells. The commonest sites of CSF leak are cribriform plate, followed by posterior wall of frontal sinus and floor of anterior cranial fossa.