All cells use double-stranded DNA as their genetic material. The accurate replication and portioning of DNA into daughter cells is a fundamental life process. A genome, all the genetic information that defines an organism, consists of one or more chromosomes and may also include plasmids. This chapter examines the structure of genomes from the molecular DNA level on up. Chromosome architecture among bacteria, archaea, and eukaryotes is compared. The mechanism of DNA replication is dissected in detail. The chapter concludes by examining DNA sequencing and the polymerase chain reaction, techniques that have helped usher in the field of metagenomics.