Viroids are the smallest infectious pathogens known, consisting solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens, some of which are of economical importance. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleobases. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleobases in size. The human pathogen hepatitis D virus is a defective RNA virus similar to viroids. Viroids, the first known representatives of a new domain of “sub-viral pathogens,” were discovered, initially characterized, and named by Theodor Otto Diener, plant pathologist at the U.S Department of Agriculture’s Research Center in Beltsville, Maryland, in 1971. The first viroid to be identified was Potato spindle tuber viroid (PSTVd). Some 33 species have been identified. Viroids do not code for any protein. Viroid’s replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes “rolling circle” synthesis of new RNA using the viroid’s RNA as template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates. With Diener’s 1989 hypothesis that viroids may represent “living relics” from the widely assumed, ancient, and non-cellular RNA world—extant before the evolution of DNA or proteins—viroids have assumed significance beyond plant pathology to evolutionary science, by representing the most plausible RNAs capable of performing crucial steps in the evolution (Abiogenesis) of life from inanimate matter.