DNA microarrays or DNA chips were introduced in the middle nineties and have developed as a very powerful tool for structural and functional analysis of genomes. With thousands to millions of probes deposited on each microarray, it is now possible to perform various kinds of analysis on the genome-wide scale. The basic use of microarrays is gene expression profiling. For this purpose, both one- and two-color labeling methods are used. More sophisticated DNA microarrays allow for analyzing alternative splicing, DNA-protein interactions, chromatine modifications and many more. Currently, DNA microarrays represent an indispensable tool in biology and medicine.