The Escherichia coli phage HK97 has provided a productive experimental system for investigating how virus capsids are assembled from their protein components and how the assembled capsids mature to their final form. Aspects of the process for which the HK97 system has been particularly informative include assembly of the icosahedral capsid shell from the component proteins, structure of the capsid subunits and of the entire capsid as it progresses through its maturation, and the mechanism of the covalent cross-linking that links the subunits together into viral chain mail. The structural dynamics of the maturation as well as the energetics that drives maturation forward are beginning to be understood.