MicroRNAs (miRNAs) are endogenous, small non-coding RNAs (20-22 nucleotides) that negatively regulate gene expression at the translational level by base pairing to the 3' untranslated region of target messenger RNAs. More than 400 miRNAs have been identified in humans and are evolutionally conserved from plants to animals. It has been revealed that miRNAs regulate various biological processes, such as development, cell differentiation, cell proliferation, and cell death. It is predicted that 30% of protein-encoding genes are regulated by miRNAs. Inappropriate expression of miRNAs has been found in cancer. Especially, the expression level of miRNAs that act like anti-oncogenes is frequently reduced in cancers because of chromosome aberrations. In addition, since the processing of miRNAs has been characterized to be enzymatic in nature, the expression levels of miRNAs are closely associated with the activity and levels of such enzymes. In this review, we discuss recent remarkable advances in miRNA biogenesis, bio-networking involving miRNAs, and their roles in carcinogenesis. Further, we discuss the expression of miRNA-143 and -145 in colon cancer and their roles in carcinogenesis. The available data suggest that miRNAs would be potentially useful as diagnostic and therapeutic tools.