The sequence of steps in bacteriophage T4 assembly has been elucidated by using a combination of genetic, biochemical, and ultrastructural techniques. The phage head, tail, and tail fibers are assembled via independent pathways, and then are jointed to form the complete virus. Current knowledge of these three pathways is reviewed briefly. Two general insights emerging from phage assembly studies are (1) a realization of the importance of kinetic controls, and (2) recognition of the role of nonstructural accessory proteins in assembly. Controls of protein association rates by a proposed heterocooperation mechanism can account for the strict sequential ordering of steps in complex self-assembly pathways such as that of T4 tail assembly. The same mechanism can explain how proteins capable of polymorphic assembly are induced to form correct structures rather than aberrant ones of similar stability. Nonstructural accessory proteins provide additional means for enhancing rates of interactions of specific structural proteins by mechanisms that may be analogous to those of enzyme catalysis. The insights gained from bacteriophage assembly probably apply to organellogenesis in general.