MicroRNAs (miRNAs) are endogenously expressed and evolutionarily conserved small non-coding RNAs, which regulate gene expression. Several studies have shown that they are involved in fundamental biological processes, such as proliferation and apoptosis. MicroRNA dysregulation plays an important role in cancer onset and progression where miRs can function as both tumor promoters (oncomiRs) or tumor suppressors by targeting numerous biomolecules that are important in carcinogenesis. MicroRNA molecules are already entering the clinic as diagnostic and prognostic biomarkers for patient stratification and also as therapeutic targets and agents. Their role as biomarkers and therapeutic targets is appealing but several obstacles have as yet limited our ability to translate this potential into a clinical reality. This review provides a comprehensive overview of miRNAs with established functional relevance in cancer. Furthermore, approaches towards therapeutic miRNA-based intervention are discussed. Those include viral or non-viral approaches of miRNA replacement therapy in the case of tumor-suppressing miRNAs and strategies for the inhibition of oncogenic miRNAs.