Genevestigator is a recently developed online analysis system for researchers and clinicians to study at what time, in which organs, and in response to which factors or diseases genes are activated. Massive amounts of microarray data (> 500 million measurements) from several organisms were collected from laboratories world-wide, quality controlled, systematically annotated, and stored in the Genevestigator database. The tools developed allow searching for biomarker genes, to build gene networks, and ultimately to design models of how genes interact to coordinate growth or how these networks act in response to diseases. A further step will be to include cancer data to facilitate basic research and diagnostics in this area.