Like all positive strand RNA viruses, the picornaviruses replicate their genomes using a virally encoded RNA-dependent RNA polymerase enzyme known as 3D. Over the past decade we have made tremendous advances in our understanding of 3D structure and function, including the discovery of a novel mechanism for closing the active site that allows these viruses to easily fine tune replication fidelity and quasispecies distributions. This review summarizes current knowledge of picornaviral polymerase structure and how the enzyme interacts with RNA and other viral proteins to form stable and processive elongation complexes. The picornaviral RdRPs are among the smallest viral polymerases, but their fundamental molecular mechanism for catalysis appears to be generally applicable as a common feature of all positive strand RNA virus polymerases.