The lifecycle of a virus begins with its entry into a host cell, followed by uncoating, the replication of nucleic acid and protein, and the assembly of a capsid. A virus then undergoes envelopment, release, and in some cases maturation. Until recently, capsid formation was believed to occur spontaneously through a process of self-assembly. Our studies show the capsids of several unrelated viruses form through a complex biochemical pathway involving discrete intermediates and host molecular facilitators.