As immunohistochemistry enters its fourth decade as a diagnostic tool, this analytical procedure continues to experience poor reproducibility, albeit in specific situations. The problem becomes more important with the mounting pressure to employ the assay in a quantitative manner for the assessment of therapeutic and prognostic markers in a number of neoplasms. Qualitative applications of immunohistochemistry are well established and continue to increase with the range of sensitive antibodies and detection systems available, however, numerous variables that influence the immunoexpression of proteins in formalin-fixed tissue continue to exist in the pre-analytical and analytical phases of the test procedure. This book emphasises these problems in detail and suggests possible solutions.