MicroRNAs (miRNAs) are small non-protein-coding RNA transcripts mainly responsible for the regulation of messenger RNA (mRNA) targets. In cancer, oncomiRs play either oncogenic or tumour-suppressive roles. In many cases, miRNAs play both roles, having context-dependent mechanisms of action. The differential expression levels of miRNAs have been associated with several types of cancer, in many cases making it possible to infer clinical correlation and prognosis, acting as biomarkers, or presenting valuable targets for the development of new therapies. In haematological malignancies, specifically in chronic lymphocytic leukaemia (CLL) and acute myeloid leukaemia (AML), several miRNAs were found to be involved in the pathogenesis and progression of the disease. In this chapter, we present the concept of miRNAs and their biogenesis, how they are dysregulated in cancer and involved in physiological haematopoiesis, followed by their implications in CLL and AML, giving the latest perspective in the field and discussing the use of miRNAs as prognostic markers and targets for CLL and AML therapy.