MicroRNAs (miRNAs) are causing tremendous excitement in cancer research. MiRNAs are a large class of short non-coding RNAs that are found in many plants, animals and DNA viruses and often act to inhibit gene expression post-transcriptionally. Approximately 500 miRNA genes have been identified in the human genome. Their function is largely unknown, but data from worms, flies, fish and mice suggest that they have important roles in animal growth, development, homeostasis and disease. MiRNA expression profiles demonstrate that many miRNAs are deregulated in human cancers. MiRNAs have been shown to regulate oncogenes, tumour suppressors and a number of cancer-related genes controlling cell cycle, apoptosis, cell migration and angiogenesis. MiRNAs encoded by the mir-17-92 cluster have oncogenic potential and others may act as tumour suppressors. Some miRNAs and their target sites were found to be mutated in cancer. MiRNAs may have great diagnostic potential for human cancer and even miRNA-based cancer therapies may be on the horizon.