Over the years numerous surveys have been undertaken to assess the incidence of skin and liver tumors, as well as other contaminant-associated microscopic lesions at areas of concerns (AOCs). However, a major problem is a lack of consistent criteria for evaluating microscopic (histological) changes in bullhead liver and skin. For instance, in some studies there is no distinction between preneoplastic (altered foci) and neoplastic lesions, or no good descriptions of individual lesions (Black 1983; Pyron et al. 2001). Some studies included only carcinomas as neoplasia, while in others adenomas, carcinomas, and cholangiomas were included (Baumann et al. 2000) and some included carcinomas, adenomas and altered foci (Maccubbin and Ersing 1991). Skin tumors are often diagnosed visually without the supporting histological verification. Hence, standardization of the criteria being used to evaluate the impairment is necessary, so that data from different years, reference sites and Areas of Concern can be adequately compared.