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Urea cycle deficiencies are inherited disorders of the ammonia detoxification pathway and arginine synthesis. They are caused by deficiencies in urea cycle enzymes. The urea cycle is the main metabolic pathway for purifying ammonia, a toxic waste product of the body’s protein catabolism. Clinical signs are therefore partly linked to ammonia intoxication. Diagnosis can be made at any age and requires the use of a range of investigative techniques, most of which are specific. The aim of our work is to review the pathophysiology and clinical features of urea cycle deficiencies, and to present the various biological diagnostic methods currently available and used to investigate these deficiencies.