Emerging evidence suggests a class of non-coding RNAs termed microRNAs (miRNAs) play a key role in cancer. Since their original discovery in C. elegans in 1993 it has become evident that miRNAs are responsible for an entirely new mechanism of post-transcriptional gene regulation. miRNA expression is widespread in mammalian cells and notably altered in several cancer types. miRNA expression patterns correlate with several aspects of tumorigenesis and miRNA loci have been mapped to frequently altered cancer-associated genomic regions. Inhibition or augmentation of miRNA expression in cancer cells impacts gene expression and affects cell proliferation and survival. Hence, cancer-associated miRNAs may be regarded as a new class of non-coding tumour suppressors and oncogenes capable of regulating several key signalling pathways.