MicroRNAs (miRNAs) are a new class of small, non-coding RNAs that regulate gene expression. The base pairing interactions between miRNAs and their target mRNAs, often within the 3'-untranslated region (UTR) of target genes, result in the degradation of target mRNAs or repression of their translation. MiRNAs regulate a diverse range of physiological processes, including cell differentiation and proliferation, mammalian development and human disease. Many studies have shown that miR-17-92 cluster, which encodes miR-17-5p, miR-17-3p, miR-18a, miR-19a, miR-20a, miR-19b-1, and miR-92-1, is expressed in many mammalian tissues. This cluster contributes to the development of heart, lung, blood vessel, and immune system. In addition, it can induce tumorigenesis, such as lymphoma and vascularized tumor as an oncogene. However, miR-17-92 cluster proved to suppress breast cancer cell proliferation and tumor colony formation as a tumor suppressor. This paper reviews the roles of miR-17-92 cluster in mammal development and the relationship between miR-17-92 cluster and tumorigenesis.