MicroRNAs (miRNAs) are a family of 21- to 25-nucleotide, noncoding small RNAs that primarily function as gene regulators. It is surprising that these tiny molecules, so diverse and consequential in their biological functions, have been hidden for so many years. Thanks to their discovery, cancer research has found a new arena. Aided by innovative molecular techniques, the research of miRNAs in oncology has progressed rapidly in recent years. miRNA abnormalities are becoming an emerging theme in cancer research. Specific functions of miRNAs, many of which are relevant to cancer development, are becoming apparent. The value of miRNAs in cancer classification and prognostication is being explored, and new therapeutic strategies targeting miRNAs are being developed. Because there is great promise that miRNA research will provide breakthroughs in the understanding of cancer pathogenesis and development of new valuable prognostic markers, pathologists should be adequately informed of this rapidly progressing field. Here, we offer a review on the basics of miRNA biology and the emerging role of miRNA in cancer pathogenesis, classification, and prognostication, including highlights of the involvement of specific miRNAs in different tumor types.