MicroRNAs (miRNAs) are short endogenous noncoding small RNA molecules with 21 to 25 nucleotides (nt) in length, which negatively regulate gene expression either by degrading specific mRNA or impeding translation at the transcriptional and post-transcriptional level. Recent advances suggest that miRNAs play prominent roles in development, genome organization, viral and transposon defense, and human disease, including neurodegenerative and metabolic diseases. Surprisingly, miRNAs are now linked with a variety of cancers, in which a reduced expression or overexpression may lead to oncogene or tumor suppressor gene-like actions. Here we will overview the miRNA pathway and its regulation with emphasis on the links with human cancer. Finally potential applications of miRNA in tumor diagnosis and therapy will be discussed.