In Chapter 10 of thts volume and in prior publications (1 ,2 ), the matertals and methods used in immunohtstochemical techniques are described, and factors that affect the immunohistochemtcal detection of biomarker expression are discussed In this chapter, our semtquantbiattve method of evaluation of btomarker expression is explained. Also, the use of nnrnunohistochemtstry (IHC) to characterize btomarker expression in neoplastic lesions of the prostate and colorectum is dtscussed. The use of btomarkers in the study of neoplasia has expanded in recent years as new uses for biomarkers have been identified. For example, our laboratory has studied biomarkers to characterize tumorigenesis, support novel therapies, aid in diagnoses, predict aggressive tumor subtypes, and monbior the response of oral dysplasta to chemoprevention wbih retmoids. In studying oral neoplasia, we found that the expression of transforming growth factor α (TGFα) was increased in leukoplakia and that treatment wbih 13-cis-retmotc acid caused complete or partial resolutton of leukoplakia and concombiant decreased expression of TGFα (3 ). Thus, TGFα could be considered a candidate surrogate intermediate endpoint btomarker for chemoprevention studtes of oral dysplasia. Similarly, we have used the evaluation of biomarker expression in tissues to qualify patients wbih tumors of the prostate, colorectum, breast, ovary, and lung for specrfic types of immuno-therapy and for immunization wbih tumor vaccines (4 ). We also have studied biomarker expression in order to characterize the putative preinvasive neoplastic lesion of the prostate, prostatic intraepbihelial neoplasia (PIN), and to relate PIN to invasive prostatic adenocarcinomas (5 –8 ). Determination of biomarker expression in colorectal tumors has been useful in identifying racial differences in these tumors as well as differences in histopathological tumor types (9 ). In this chapter, we demonstrate the advantages of semiquantbiatively evaluating biomarker expression in neoplastic lesions of the prostate and colorectum.