Research during the last 15 years has conclusively shown that viroids are not 
only fundamentally different from viruses at the molecular level, but that they 
are most likely not directly related to viruses in an evolutionary sense. Today, 
viroids are among the most thoroughly studied biological macromolecules. Their 
molecular structures have been elucidated to a large extent, but much needs to 
be learned regarding the correlation between molecular structure and biological 
function. The availability of the tools of recombinant DNA technology in viroid 
research promises rapid progress in these areas of inquiry.