Whether polyclonal or monoclonal antibodies should be used in immunohistological staining is sometimes a matter of personal preference. Most often, however, the type of antigen and the tissue to be studied will influence this selection. All prepared and purified antibodies must be submitted to quality and specificity testing. Proteome microarrays containing proteins from the relevant organism and microarrays with unrelated proteins represent an ideal format for an assay to prove antibody specificity. This will allow the simultaneous screening of thousands of proteins for possible cross-reactivity. Tissue microarrays appear to be greatly useful as validation tools and, generally, in biomarker research. By arraying multiple normal tissues and different tumor types, one can analyze molecular targets and their histological distribution. Also, staining patterns of multiple antibodies are easily compared and evaluated for specificity.