Viroids are infectious, nonprotein-coding, highly structured small circular ribonucleic acids (RNAs) able to replicate autonomously and induce dis- eases in higher plants. Viroids and viruses dif- fer in structure, function and evolutionary origin (with the former regarded as relics of a primitive RNA world). Viroids are classified into the fami- lies Pospiviroidae and Avsunviroidae, the members of which replicate in the nucleus and chloroplast, respectively, through an RNA-based rolling circle mechanism with three steps catalysed by (1) host deoxyribonucleic acid (DNA)-dependent RNA poly- merases redirected to accept RNA templates, (2) ribonucleases or, in the Avsunviroidae, hammerhead ribozymes and (3) RNA ligases, either DNA ligase 1 redirected to accept RNA substrates in the Pospivi- roidae or a tRNA ligase in the Avsunviroidae. When infecting a cell, the viroid RNA must travel to its eplication organelle, with the resulting progeny moving cell-to-cell through plasmodesmata and reaching distal parts through the phloem. Patho- genesis could be triggered by the replicating viroid itself, or via RNA silencing. Viroids are detected by nucleic acid-based approaches, some can be elim- inated from infected plants by meristem tip cul- turing, and biotechnological approaches are being developed for their control.