MicroRNAs (miRNAs) are small non-coding RNAs that downregulate gene expression during various crucial cell processes such as apoptosis, differentiation and development. Recent work supports a role for miRNAs in the initiation and progression of human malignancies. Moreover, large high-throughput studies in patients revealed that miRNA profiling has the potential to classify tumours and predict patient outcome with high accuracy. Functional studies, some of which involve animal models, indicate that miRNAs act as tumour suppressors and oncogenes. This review examines the role of miRNAs in the pathogenesis of cancer as well as miRNA-profiling studies performed in human malignancies. Implications of these findings for the diagnosis and treatment of cancer patients are also discussed.