In situ hybridization (ISH) is the demonstration of specific genetic information within a morphologic context. For HPV, colorimetric ISH has the advantage that it can be applied to routine formalin-fixed paraffin embedded tissues. This conserves patient material and permits histologic selection of optimal material for testing. ISH allows for precise spatial localization of viral sequences within tissues. ISH also allows the integration status of HPV to be determined. The major limitations of the method are the potential for error in HPV typing because of probe cross-hybridization and relatively low sensitivity if the method is not fully optimized.