Bacteriophage T7 is a virulent phage that infects E. coli. The seventh of the set of T phages focussed on by the Cold Spring Harbor group of phage workers in the 1940's, T7 has been the object of considerable genetic and biochemical investigation. The T7 virion contains a single molecule of linear, double-stranded DNA, and the nucleotide sequence of the entire 39,936 base pairs has now been determined. All of the known T7 genes and genetic signals have been located within the nucleotide sequence, and several previously unsuspected genes have been revealed. Our current understanding of the genetic organization of T7 DNA and the overall pattern of gene expression during T7 infection has been discussed in detail in relation to the nucleotide sequence. The present article is a condensation of a more general review of the organization and expression of T7 DNA, and is meant to give an overview and introduction to the system.