The presentation focuses on the structural rearrangements of the subunits and the processing of the various protein constituents which accompany the maturation events of the head of bacteriophage T4. The major features of the maturation steps of the head are the following: (a) the viral DNA is pulled into an empty head in a series of events; (b) cleavage of two core proteins, P22 (mol. mass = 31000), to small fragments and the internal protein IPIII (mol. mass = 23000) to IPIII (mol. mass = 21000) appears to be intimately linked to the DNA packaging event, whereas the cleavage of the major head protein of the viral coat, P23 (mol. mass = 55000), to P23 (mol. mass = 45000) precedes the DNA packaging event. Recently, we have obtained information about the mechanism by which the viral DNA is pulled into a preformed empty head. Our evidence suggests that the DNA becomes attached to the inside of the empty head and is subsequently collapsed in the interior by the so-called internal peptides. These are highly acidic and derived from a large precursor protein by cleavage.