Viroids are small, circular, single-stranded RNA molecules that cause several 
infectious plant diseases. Viroids do not encode any pathogen-specific peptides 
but nonetheless, the subviral pathogens replicate autonomously and spread in the 
plant by recruiting host proteins via functional motifs encoded in their RNA 
genome. During the past couple of years, considerable progress has been made 
towards comprehending how viroids interact with their hosts. Here, we summarize 
recent findings on the structure-function relationships of viroids, their 
strategies and mechanisms of replication and trafficking, and the identification 
and characterization of interacting host proteins. We also describe the impact 
of the RNA silencing machinery of plants on viroid RNAs and how this has started 
to influence our models of viroid replication and pathogenicity.