Publisher Summary This chapter provides an overview of the virus of foot-and-mouth disease (FMD). It discusses the existence of different immunological types of the virus of FMD. It also discusses the methods of typing, the geographical distribution of virus types, and nature of the type difference in the virus of FMD. Although this review deals especially with the virus and not with the disease, a brief discussion on vaccines and vaccination is included. The chapter describes the tests for noninfectivity. As with all inactivated viral vaccines, the first test is that for noninfectivity. In dealing with tests on the virus of FMD, it has been shown that the intradermal tongue route is more sensitive than other routes for detection of virus. The other advantage of testing by the intradermal (tongue) route are: (1) that the presence of noninfective but antigenic material in the inoculum does not hinder the detection of a trace of active virus; (2) multiple observations can be obtained from one animal; and (3) the initial susceptibility of the tongue tissue of nonreactors can usually be determined at the end of the test.