MicroRNAs (miRNAs) can be defined as small, noncoding sets of 19 to 24 nucleotides that have been associated with messenger RNA expression. miRNAs are members of a class of small regulatory RNAs that includes small interfering RNAs (siRNAs). miRNAs regulate the expression of downstream gene targets, including transcription factors, oncogenes, and tumor suppressor genes. Transcriptional profiling using genomic microarrays and beads has enabled the discovery of numerous miRNAs that are differentially expressed in normal tissues vs tumors and associated with cancer development, diagnosis, and prognosis. miRNA signatures can be used to detect and classify cancer and predict the severity of disease, with certain profiles of miRNA expression linked to aggressive cancers with advanced disease present at diagnosis. miRNAs have also become targets of novel anticancer gene therapy with antisense molecules that can inhibit miRNA activity currently being tested for their efficacy in a strategy of reducing miRNA activity on reporter genes bearing miRNA-binding sites. In the future, sophisticated genomic and proteomic techniques combined with complex bioinformatics data analyses will be required to translate these recent basic science discoveries to clinically useful diagnostic tests.