DNA sequencing has provided a wealth of information on the genetic material stored in bacterial genomes. The use of DNA arrays and proteomics will transform the scale of our ability to describe the patterns of gene expression as bacteria respond to their environments. However, the ability to control bacteria in a clinical context or exploit them in industrial or environmental contexts also depends on understanding the regulatory mechanisms which connect input experience to output response at the genetic level. This book deals with our current knowledge of the circuits and networks that govern bacterial gene expression - from the single gene to the whole genome - and which provide the framework for explaining the data from the post genomics revolution.