DNA replication is a key event in the cell cycle. Although our knowledge is far from complete and many elusive regulatory mechanisms still remain beyondour grasp, many enzymes and a multiplicity of biochemical mechanisms involved have been discovered. Recent findings in E. coli have confirmed and yet surpassed the original hypothesis of F. Jacob. In yeast and higher eucaryotes, the apparent redundancy in putative origins and initiators has made an estimation of the importance of each identified element difficult to access. In spite of well established methodologies - which are also described in the book - the origin identification in mammalian chromosomes is still a controversial subject. On the other hand, considerable advances have been made in our understanding of virus DNA replication and this continues to deepen and broaden our understanding of the controls of cellular DNA replication.