MicroRNAs (miRNAs) are small noncoding RNAs that suppress the abundance of partially complementary mRNAs and inhibit their translation. Each miRNA can regulate hundreds of mRNAs, sometimes strongly but often weakly, to mediate a diverse array of biological functions, including proliferation, cell signaling, differentiation, stress responses and DNA repair, cell adhesion and motility, inflammation, cell survival, senescence, and apoptosis, all intimately related to cancer initiation, treatment response, and metastasis. The expression and activity of miRNAs are spatially and temporally controlled. Global miRNA expression is reduced in many cancers. In addition, the expression and processing of cancer-related miRNAs that act as oncogenes ("oncomiRs") or tumor suppressors are often dysregulated in cancer. In this review, we summarize emerging knowledge about how miRNA biogenesis and gene silencing are altered to promote cancer.