Arrays provide a global and systematic way to survey DNA and RNA variation. They are easy to work with, especially when armed with a suitable data analyzing software. Arrays seem likely to become a standard tool of molecular biology research and of clinical diagnostics. Clinical use of microarray technology is still in its infancy and remains exploratory. The results in the sphere of tumor research and drug resistance are so promising that micro­array techniques must become an integral part of the daily activities of the typical molecular biology laboratory. We are optimistic that over the next few years the pattern of development and use of microarrays will be on a similar trajectory to that seen for computers and other high-tech electronic devices, which started out as exotic and expensive tools in the hands of a few developers and then moved quickly to become easier to use, more available and less expensive.