The classical pathological diagnosis is of decisive importance for adequate therapy choice. Unfortunately, inconsistency in judgement of the same case and disagreement in diagnosis between different pathologists can be pronounced. Nevertheless, techniques of quantitative pathology largely improve the reliability of diagnosis because of their, in principle, high objectivity and reproducibility. Besides stereology, morphometry and flow cytometry recently also digital image processing, three dimensional laser scan microscopy and multimedial integrated image-database expert systems are becoming increasingly applicated. However, to gain maximum profit of these quantitative methods, sufficient measurement error handling is indispensable. In addition, concepts of standardisation and ongoing quality control should be available to assure confident measurements as a valuable aid in the diagnostic process.