In spite of considerable advances during the last years, classification and identification of Actinomycetaceae and Propionibacteriaceae still present major problems. Uncertainties about the systematic position and taxonomic status of several genera and species, the lack of reliable distinguishing characteristics or use of inadequately adapted techniques of investigation interfere with both micribiological and clinical research and also with routine work in the diagnostic laboratory. Nevertheless, besides actinomycosis, an etiologically well defined suppurative infection, several other diseases and lesions such as lacrimal canaliculitis, caries and periodontal disease, inflammation following use of intrauterine pessaries, various types of abscess, septicaemia, and acne vulgaris have been attributed to the direct or indirect pathogenic effects of fermentative actinomycetes, propionibacteria, or eubacteria. Many questions concerning their individual pathogenicity and their pathomechanisms still remain to be answered, however.