MicroRNAs (miRNAs) represent a class of small, noncoding RNAs. These small RNAs are involved in diverse biological processes, and it has been predicted that about one third of human messenger RNAs (mRNAs) appear to be miRNA targets, underlying the major influence of miRNAs on almost all cellular pathways. Deviation from the normal pattern of miRNA expression has been implicated in several diseases. Among human diseases, it has been shown that changes in miRNA expression correlate with various human cancers. Thus, miRNA profiling could contribute to more precise tumor classification and better prediction of the therapeutic outcome. This chapter summarizes these recent findings and highlights the technical advances in miRNA probe preparation, miRNA expression profiling and target identification.