In the year 2000, the Human Genome Project Consortium presented the first complete draft of the human genome together with Celera Genetics. Since then, the so-called "post-genome era" has started. Microarrays are capable of profiling gene expression patterns of tens of thousands of genes in a single experiment and thus allow a systematic analysis of DNA and RNA variation. They seem likely to become a standard tool of both molecular biology research and clinical diagnostics. These prospects have attracted great interest and investment from both the public and private sectors. This review introduces the principle of microarray technology and gives an overview of its current and future potential in clinical medicine.