Paper reviewed the literature including current knowledge on the plant viroids. Viroids are subviral plant pathogens. They are composed solely by a single-stranded circular RNA of 246-401 nt of a compact secondary structure with the few exceptions such as rod-like or quasi rod-like conformations. Three plant viroids – ASBVd, ChCMVd and PLMVd appear to form some branched conformations. In contrast to viruses, viroids do not code for any protein, depend on host enzymes for their replication, which for some viroids occurs in the nucleus and for the others in chloroplast, through a rolling-circle. Viroids are regarded as parasites of the transcription system. Molecular techniques based on PAGE, hybridization with radioactive and chemically labelled cDNA or cRNA probes, and RT-PCR as well as bioassay are used for viroids detection.