Modern expression-screening platforms such as complementary DNA (cDNA) arrays allow for high-throughput lead discovery in cancer and other diseases. For evaluation of promising candidate genes, however, in situ analysis of high numbers of clinical tissues samples--for example, by immunohistochemistry or fluorescence in situ hybridisation--is mandatory. Tissue microarray (TMA) technology greatly facilitates such analysis. Minute tissue cores (diameter 0.6 mm) are removed from up to a thousand different conventional paraffin blocks and re-assembled in a single empty paraffin block at predefined positions. Sections of the resulting TMA can be utilised for the range of research applicable to conventional tissue sections. Important advantages of the TMA technology are speed (parallel analysis of up to a thousand tissues), cost efficiency (the same amount of reagents required for a single large-section analysis is sufficient for a thousand samples), and standardisation (the same experimental conditions are applied to all samples). Because of the high numbers of samples usually included in TMAs, they are optimally suited to detect genotype-phenotype associations with high statistical power. Thus, TMA technology will markedly accelerate the transition from basic research to clinical applications.