Viroids are low-molecular weight covalently closed circular RNAmolecules that can cause economically important diseases of higher plants. Viroids are distinguished from viruses by the absence of a protein coat, lack of mRNA activity, and by the homogenous structure, structural transitions, and hydrodynamic behavior of their RNA molecules. In the opinion of some investigators, viroid diseases are of fairly origin and their appearance is attributed to the introduction of intensive methods of agriculture, especially monocultures. Purified preparations of viroid contain variable amounts of linear molecules in addition to the circular ones and there has been some dispute about infectivity of the linear molecules. Among the viroids, avocado sunblotch viroid (ASBV) and coconut cadang-cadang viroid (CCCV) would be the less developed ones and the Potato spindle tuber viroid (PSTV) group, with its wide host range, the best developed.