Incidence, pathogenesis of fluid and electrolyte imbalance in head injury along with clinical presentation are given in detail. Investigations in a case of fluid and electrolyte imbalance in head injury are described with the help of well illustrated tables. Management, prognosis and complications of fluid and electrolyte imbalance in head injury patients are described at the end of the chapter. Hyponatraemia, hypernatraemia, cerebral salt wasting syndrome and diabetes insipidus are described comprehensively. 45 references are given for further reading to conclude this chapter.