Azo dyes play an important role as coloring agents in the textile, food, and pharmaceutical industry. Due to the toxicity, mutagenicity and carcinogenicity of azo dyes and their breakdown products, their removal from industrial wastewaters has been an urgent challenge. Promising and cost-effective methods are based on their biodegradation, which is treated in this volume. The topics presented by experts in the field include: the classification of azo dyes; toxicity caused by azo dyes; aerobic and anaerobic azo dye biodegradation mechanisms; the role of bacteria, fungi, algae and their enzymes in biodegradation; the impact of redox mediators on azo dye reduction; the integration of biological with physical and chemical processes; the biotransformation of aromatic amines; reactor modelling for azo dye conversion; the biodegradation of azo dyes by immobilized bacteria and fungi; and factors affecting the complete mineralization of azo dyes.