Abstract Knowing that the pathway of phage particle morphogenesis is not under transcriptional control, we elaborate on mechanisms acting on the level of proteins. We give examples which show one of the mechanisms by which freely available protein subunits become involved when the proteins of a precursor particle have reached a certain conformation. Through this conformational change binding sites for an additional protein become suddenly available. We also show that the form of preheads in bacteriophage development is determined in a large part by the scaffolding action of their proteineous cores. We discuss the present knowledge on form determination in connection with different theories. In addition we summarize the presently available facts and speculations in regard to the DNA packaging event.