Poliovirus was first isolated nearly 100 years ago in a landmark experiment that established the viral etiology of poliomyelitis. This discovery stimulated investigation of the pathogenesis of poliomyelitis in many laboratories. Nearly 50 years later, when two effective poliovirus vaccines were developed, the impetus to study poliovirus pathogenesis waned. The identification of the cell receptor for poliovirus, CD155, and its use in the development of transgenic mice susceptible to poliovirus revived interest in understanding how the virus causes disease. Experiments in CD155 transgenic mice have provided new information on the initial sites of virus replication in the host, how the virus spreads to the central nervous system through the blood and by axonal transport, the determinants of viral tropism, and the basis for the attenuation phenotype of the Sabin vaccine strains. Despite these advances, our understanding of poliovirus pathogenesis is still incomplete. The dilemma is not how to answer the remaining questions, but whether there will be sufficient time to do so before global eradication of poliomyelitis leads to cessation of research on the disease.