A new type of noncoding short single-stranded RNA molecules, microRNA (miRNA), has recently emerged as one of the most important new classes of cellular regulators. Recent advances in miRNA research have provided evidence that aberrant expression of specific miRNAs is associated with a broad spectrum of human diseases, such as cancer, diabetes, cardiovascular diseases, psychological disorders and others. Global microRNAome profiling has demonstrated drastic changes in expression of multiple miRNAs in many common human cancers. miRNA expression signatures have been shown to provide a more accurate method of classifying cancer subtypes than transcriptome profiling of an entire set of known protein-coding genes. miRNA profiling also allows classification of different stages in tumor progression and can predict disease outcome in some cases. Further characterization of these abnormal miRNA expression signatures and identification of the most significant and informative aberrantly expressed miRNAs could lead to the development of tissue- and biofluid-specific diagnostic markers, as well as a new type of oligonucleotide-based therapeutics. As researchers continue to study miRNA expression in cancer and focus on most relevant miRNAs, the further advancement of miRNA-detection technologies and bioinformatics algorithms is critical for successful identification of the most informative diagnostics markers among the tissue-specific miRNAs.