miRNAs are small non-protein coding transcripts that regulate gene expression post-transcriptionally by binding to the 3' UTRs (untranslated regions) of messenger RNAs (mRNAs). The number of newly discovered miRNAs and our understanding of their biological roles continue to grow. In addition to their roles in important biological processes such as development, differentiation, proliferation and cell death, deregulated expression of miRNAs has been implicated in a wide range of pathologies, especially in cancer. We now understand that miRNA expression is often deregulated in cancer cells and that a vast number of genes, including tumor suppressor genes and oncogenes, are regulated by these small RNAs. The small size of miRNAs and sequence similarity of miRNA family members pose some challenges in routine molecular detection and quantification techniques. Therefore, methods are being modified to specifically and sensitively detect miRNAs in cancer cells. Our current knowledge and the ever increasing pace of new discoveries clearly show that miRNAs are quite important in normal and in cancer cells in surprisingly diverse aspects. The better we understand how miRNAs contribute to cancer, the more likely we will be able to exploit them as tumor classifiers, biomarkers and, potentially, as unique targets for therapeutic applications.