MicroRNA (miRNA) cluster is a set of two or more miRNAs, which are transcribed from physically adjacent miRNA genes. MiRNA genes can be found either in protein-coding or non-coding regions of transcription units (TUs). Except for the physically adjacent transcription location, three criteria are required to restrict the definition of miRNA cluster: miRNAs in a cluster are transcribed in the same orientation, and are not separated by a transcription unit or a miRNA in the opposite orientation. Usually, miRNA clusters consist of two or three miRNAs, but larger clusters also exist, for example, the miR-17-92 cluster is found on human chromosome 13 and is composed of six miRNAs. miRNA clusters play important roles by controlling various cellular processes. For example, the most frequently studied miR-17-92 cluster is involved not only in tumor formation but also in the development of heart, lungs, and immune system.