MicroRNAs (miRNA) are tiny bits of genetic material that were unknown nearly 10 years ago but now represent an exciting field of study in biology. Upon their discovery, researchers revealed for the first time a new mechanism by which microRNA can stop the function of messenger RNA (mRNA) by literally cutting it in half, interfering with the normal function of specific messenger RNAs in gene expression. This "expression" of genes that code for essential proteins is essentially what controls whether a cell turns into a liver, lung, or brain cell, for example. Understanding what activates this process – or stops it – is a key to understanding the biological process and builds a foundation for advances in medicine and other fields. This volume in Methods in Enzymology presents valuable methods for studying MicroRNA, with three sections covering identification of MicroRNAs and their targets; MicroRNA expression, maturation and functional analysis; and MicroRNAs and disease.