In 1971, unique small RNA molecules, the viroids, were found to cause specific infectious diseases of plants. They are the smallest and simplest contagious agents known. Until now, 14 viroids have been described and 12 diseases of potatoes, tomatoes, citruses, chrysanthemums, cucumbers, hops, coconut palms avocado trees and burdock are known to be caused by viroids. The common symptoms of these diseases are: stunting of plants. discoloration of veins, epinasty, curling and distortions of leaves, chlorotic or necrotic spots etc., followed by death of the diseased plants. All viroids are ssRNAs of m.w. ranging from 1.1 x 10(5) to 1.7 x 10(5), corresponding to chains of just 246 to 371 ribonucleotides. For 10 viroids, complete nucleotide sequences are known PSTV, CSV, CEV, TPMV and TASV show 60%-80% homology with each other; in analogy, ASBV, HSV, CPFV. GV and CCCV are closely homologous to each other, too, but just distantly related to the PSTV group. Extensive intramolecular base pairing creates a characteristic secondary structure of the cyclic viroid RNA chain, native viroids appearing as quasi double-stranded, unbranched, very short rod-like structures with short single-stranded loops. (Thus PSTV forms rods about 50 nm long and 2 nm wide.) The stretch of nearly all viroids bears a common central conserved region of 19 bp. The "upper" part of this region is, presumably, the cleavage-ligation site of viroid oligomers during replication. Viroids are located and replicated in nuclei of infected cells, in association with their nucleoli. Their replication is directed by host DNA-dependent RNA polymerase II using cRNA oligomers as templates according to the rolling circle model. Viroid RNA has no mRNA function. The virulence of viroids is coded by their virulence modulating region in the "left hand" part of their molecules: a single nucleotide substitution between nucleotides 43 and 56 within this region alters the virulence. Most probably, viroids have originated by the circularization of spliced-out transcripts of eucaryotic introns. A stable complex may be created between the 5' end of U1 snRNA and nucleotides 257 to 279 of PSTV cRNA strand; thus the pathogenic effects of viroids seem to be a result of their interference with pre-mRNA processing.