RNA-dependent RNA polymerases (RdRPs) play central roles in both transcription and viral genome replication. In picornaviruses, these functions are catalyzed by the virally encoded RdRP, termed 3D. Polymerase 3D also catalyzes the covalent linkage of UMP to a tyrosine on the small protein VPg. Uridylylated VPg then serves as a protein primer for the initiation of RNA synthesis. Seven different crystal structures of foot-and-mouth disease virus (FMDV) 3D catalytic complexes have enhanced our understanding of template and primer recognition, VPg uridylylation, and rNTP binding and catalysis. Such structural information is providing new insights into the fidelity of RNA replication, and for the design of antiviral compounds.