MicroRNA (miRNA) is a major family of small RNAs that posttranscriptionally regulate gene expression. Small RNA profiling studies have revealed that some viruses, particularly large DNA viruses, such as Marek's disease virus (MDV), encode their own set of miRNAs. There are currently 406 viral miRNAs in miRBase, of which 392 are encoded by herpesviruses. To date, 26 MDV-1 miRNAs, 36 MDV-2 miRNAs, and 28 herpesvirus of turkeys miRNAs have been identified. Interestingly, herpesvirus miRNAs appear to have spatial conservation, located in clusters within repeat regions, but lack sequence conservation. Two clusters of MDV-1 miRNA have been identified, one located near the MEQ gene and one within the latency-associated transcript (LAT). miRNA profiling studies have shown that MDV miRNA are differentially expressed between strains and stages of infection. For example, mdv1-miR-M4 and mdv1-miR-M2-3p are three- and sixfold higher, expressed, respectively, in vv+ strains compared to vv strains. A recent study found that deletion or seed region mutation of mdv1-miR-M4 reduces viral oncogenicity, suggesting a link between mdv1-mir-M4 and lymphoma development in MDV-infected birds. Taken together, current research suggests that viral miRNAs are a key component of MDV pathogenesis.