Transcriptional profiling using DNA microarrays has proved to be a valuable tool for dissecting bacterial adaptation to various environments, including human hosts. Analysis of genomes and transcriptomes of Pseudomonas aeruginosa shows that this bacterium possesses and expresses a core set of genes, including virulence factors, which allow it to thrive in a range of environments. Transcriptional regulators previously thought to control single virulence traits are now shown to regulate complex global signaling networks. Microarray-based research has led to the discovery of upstream regulators and downstream components of these pathways, as well as probed the response to antibiotics, environmental stresses and other bacteria. Independent studies have highlighted the role of media composition, the makeup of the physical environment and experimental methods in the outcome of microarray analyses. A compilation of all the published data clearly shows transcriptional regulation of genes in all functional classes. Under conditions examined to date, slightly more than a quarter of the genome is regulated, suggesting that P. aeruginosa may use much of its genome for conditions unexplored in the laboratory.