Citric acid is traditionally produced by fermentation of moulds since the beginning of the 20th century. However, its overall demand continually increases due to its very widespread applications ranging from traditional food use to more specific pharmaceutical and more recent biopolymer uses. Therefore the production of citric acid is also continually improved involving the most recent tools and techniques of the field. This book discusses the metabolism of citric acid production by Aspergillus niger; biotechnological productions of citric acid; citrus fruits compounds and ageing; occurrence and metabolism of citric acid by lactic acid bacteria from fruits and fermented beverages; and citric acid treatment of cancer.