Aberrant mi(cro)RNA expression is associated with most pathological disease processes, including carcinogenesis. The knowledge that miRNAs have dual roles, as oncogenes or tumour suppressors, unveils their remarkable potential in novel cancer therapeutics. Additionally, the ability of miRNA expression profiles to classify tumours according to clinico-pathological variables highlights their potential as cancer biomarkers. This may contribute to improved patient selection for adjuvant therapies and help monitor response to treatment. MiRNAs may be exploited as therapeutic agents in various ways: firstly through antisense-mediated inhibition of over-expressed miRNAs, secondly through 'replacement' of under-expressed miRNAs with either miRNA mimetics or viral vector-encoded miRNAs, and thirdly by modulating miRNA expression to augment a patient's response to existing treatments. This chapter will outline the specific mechanisms and applications for each of these therapeutic strategies. We will also outline the progress achieved to date in this field, and the challenges that remain to be addressed before miRNA-based therapies become the next generation of cancer treatments.