MicroRNAs (miRNAs) are an abundant class of approximately 22-nucleotide – noncoding RNAs, which play important regulatory roles in animal and plant development: they are involved in gene expression at the posttranscriptional level by degrading or blocking translation of messenger RNA (mRNA) targets. miRNAs can induce RNA cleavage and chromatin modifications, and are implicated in apoptotic pathways and regulation of cell growth and proliferation. It is becoming clear that miRNAs play important roles in the regulation of gene expression during development, and our knowledge of the expression levels or function of miRNAs in normal and neoplastic cells is increasing. Accumulating experimental evidence suggests that different miRNAs are deregulated in primary human tumors and that many human miRNAs are located at genomic regions linked to cancer. miRNAs may be important regulators of mammalian hematopoiesis. They are involved in a variety of hematological malignancies, including acute lymphoblastic leukemia, acute myeloid leukemia, chronic myelogenous leukemia, chronic lymphocytic leukemia, diffuse large B-cell lymphoma, and primary effusion lymphoma. Here, we provide background on the biogenesis and function of miRNAs and discuss potential therapeutic applications of miRNA-based technology in hematological malignancies.