The study of plant viruses and their interaction with the plant host has contributed greatly to our understanding of plant biology. The recent development of plant viruses as transient expression vectors has not only enhanced our understanding of virus biology and antiviral defense mechanisms in plants, but has also led to the use of plant viral-based vectors as tools for gene discovery and production of recombinant proteins in plants for control of human and animal diseases. An overview of the state-of-the-art of viral expression systems is presented, as well as examples from our laboratory on their use in identifying nuclear targeting motifs on viroid molecules and development of therapeutic proteins for control of animal diseases.