A large portion of the human genome was long considered to be ‘junk’ or non-coding DNA. The discovery of the regulatory potential of non-coding RNAs has resulted in the differentiation of ‘junk’ DNA with unknown functions (pseudogenes) from non-protein coding DNA, including structural non-coding DNA (centromeres), functional non-coding RNAs (ribosomal RNAs) and regulatory non-coding RNAs, such as microRNAs (miRNAs). Currently, the main focus of miRNA research is related to the roles of miRNAs in cancer development. The biogenesis and modes of action of miRNAs have not been completely elucidated; however, miRNA-mediated translational repression is involved in the regulation of almost every cellular process. Thus, pathological alterations in miRNA expression signatures are commonly associated with pathological health conditions. This review specifically focuses on miRNAs in cancer, with an emphasis on their use as potential biomarkers for cancer diagnosis and prognosis. In addition, we discuss the potential use of synthetic antisense or miRNA mimetic oligonucleotides and dietary agents to modulate miRNA expression for chemotherapy and chemoprevention of cancer, respectively.