Bacteriophage T4 is a species of bacteriophages that infect Escherichia coli bacteria. It is a double-stranded DNA virus in the subfamily Tevenvirinae from the family Myoviridae. T4 is capable of undergoing only a lytic life cycle and not the lysogenic cycle. The T4 bacteriophage has a contractile tail, which is responsible for recognizing and puncturing the bacterial host and transferring the viral genome into the host during infection. T4 is a well-studied, obligately lytic phage of E. coli. It can efficiently bind to, infect, and kill E. coli in stationary phase, both in the presence and absence of a nutrient source. T4 can appear to lie relatively dormant in the cell, in what we call “hibernation” mode. Here, it initiates the infection process but produces no progeny phage until appropriate fresh nutrients are added, whereupon it rapidly makes large numbers of phage. T4 has evolved a mechanism that allows infected cells to detect related extracellular phages attempting to super-infect the cell and delay the lysis of phage-pregnant cells to release progeny for about 6 h, giving T4 an advantage where there are substantially more phage around than susceptible hosts.