MicroRNAs (miRNAs) have emerged as a class of broadly conserved small RNAs that facilitate the sequence-specific, post-transcriptionally regulation of gene expression. These small regulatory RNAs have been shown to play essential roles in many important biological processes from metabolism to apoptosis. Alterations in miRNA expression profiles can have pathogenic consequences. This article will examine the role that miRNAs play in the pathophysiology of viral infections and cancer