MicroRNAs (miRNAs) are evolutionarily conserved small non-coding RNAs that regulate gene expression. Early studies have shown that miRNA expression is deregulated in cancer and experimental data indicate that cancer phenotypes can be modified by targeting miRNA expression. Based on these observations, miRNA-based anticancer therapies are being developed, either alone or in combination with current targeted therapies, with the goal to improve disease response and increase cure rates. The advantage of using miRNA approaches is based on its ability to concurrently target multiple effectors of pathways involved in cell differentiation, proliferation and survival. In this Review, we describe the role of miRNAs in tumorigenesis and critically discuss the rationale, the strategies and the challenges for the therapeutic targeting of miRNAs in cancer.