The art of dyeing textiles has a long history, as natural dyes have been used since prehis-toric times. With the development of synthetic dyes in the 19th century, the focus shifted from natural to synthetic dyes due to their superior properties. Recently, however, interest in natural dyes has increased again due to environmental and health concerns. Among industrial dyes, heterocyclic dyes, especially azo dyes, are of great importance due to their color brilliance and fastness. This review examines the synthesis, application and analysis of azo dyes, especially heterocyclic dyes. It deals with monoazo, diazo and polyazo dyes and highlights their structures, synthesis methods and fastness properties. In addition, the ecological impact of azo dyes and practical solutions for their synthesis and application are discussed.