In situ hybridization (ISH) is one of the molecular techniques that has applications in diagnostic hematopathology. This procedure allows the detection of DNA or RNA in intact cells from various preparations, including cytology specimens and routinely fixed paraffin-embedded tissues. ISH is therefore analogous to detecting proteins (antigens) in intact cells with immunohistochemistry. The purpose of this article is to review the basic concepts and principles of ISH and to briefly discuss the important technical details of this procedure. Examples of potential applications of ISH in hematopathology are then discussed, including detection of Epstein-Barr virus, Y chromosome, and oncogene activation.