The discovery of microRNAs (miRNAs) is a remarkable breakthrough in the field of molecular genetics, as miRNAs are key actors which regulate gene expression in diverse cellular processes from unicellular yeast to human. The recent discovery of virus-encoded miRNAs indicates that viruses also use this fundamental mode of gene regulation. Research into viral miRNAs function demonstrates that some miRNAs play an important role in regulating both the viral life cycle and the interaction between viruses and their hosts. The first in vivo "antagomir" study provides an exciting first step towards miRNA therapy, and the potential for ultimately designing molecular medicines based on the modulation of miRNAs seems good.