This chapter discusses the genetic diversity of some mammalian reoviruses and considers the structure of reoviruses and function of reovirus genes encoding outer capsid proteins. It presents the genetic diversity among various isolates and how it is being defined at the nucleic acid level. Viral tropism, the specificity of a virus for a particular host tissue, is determined in part by the interaction of viral surface structures with cell-surface receptors on host cells. viruses that have an outer protein shell surrounding an inner shell (or "core"), almost all of the proteins whose functions have been found to be related to interaction with the environment and the extracellular sites in the host, are proteins found in the outer capsid. A variety of mutants have been identified for the reoviruses. In addition, there has been extensive analysis concerning the nature of reversion of the mutants. The mutants are temperature-sensitive mutants, deletion mutants, and extragenic suppression type mutans.