MicroRNAs (miRNAs) are non-coding RNAs that regulate protein expression. Aberrant miRNA expression in cancer has been well documented; miRNAs can act as oncogenes or tumor-suppressor genes, depending on the cellular context and target genes that they regulate, and are involved in tumor progression and metastasis. The potential mechanisms by which miRNAs are involved in tumor aggressiveness include migration, invasion, cell proliferation, epithelial-to-mesenchymal transition, angiogenesis and apoptosis. MiRNAs are involved in various cellular pathways and an miRNA can elicit more than one biological effect in a given cell. Existing data show the potential clinical utility of miRNAs as prognostic and predictive markers for aggressive and metastatic cancers. The stability of miRNAs in formalin-fixed, paraffin-embedded tissues and body fluids is advantageous for biomarker discovery and validation. In addition, miRNAs can be extracted from small biopsy specimens, which is a further advantage. Finally, miRNAs are potential therapeutic agents for personalized cancer management.