This article summarizes the medical, ecological, industrial and biotechnological importance of actinomycetes. At present, according to a new hierarchic classification system, they are the most representative group of Actinobacteria classis nov. These unusual microorganisms with such rich biological activity have had a confusing history and classification for over a hundred years. In this review, following subjects have been raised: important, but less known human, animal and plant pathogens as well as actinomycete-diseases; symbionts as endophytes in nitrogenase activity; agents of biological control; biodeterioration and biodegradation; primary and secondary metabolites including their biological, industrial and biotechnological importance. Additionally, some lipid compounds of cell wall as immunomodifiers have been also mentioned. Actinomycetic activities have always had far-reaching effects, and these are sure to remain in different guises.