The up-to-date knowledge on the formazan structure in crystals and solutions is considered, the problems of tautomeric and conformational equilibria are discussed. New groups of formazans synthesized over the last 10 years are presented. The results of structural studies of formazan complexes with various coordination modes of the metal are generalised. Examples of the preparation of formazan-containing polymers are given. The attention is concentrated on the analytical applications and practical use of formazan derivatives.