Technical Abstract: Phenazine compounds produced by certain species of bacteria have antibiotic activity against a wide range of bacterial and fungal pathogens including many that cause important root diseases of plants. The antibiotic activity of these compounds has long been known but the mechanism of synthesis is poorly understood, making it difficult to fully exploit their biological potential. This paper reviews the discovery and early research on phenazines, the roles of these compounds in nature, and the genetics, enzymology and biochemistry of phenazine biosynthesis. Molecular mechanisms of phenazine ring assembly are presented and unresolved questions are considered about the biosynthesis, regulation, resistance mechanisms and physiological roles of phenazines.