Classical studies of virus disease resistance in plants have provided the basis for recent molecular studies of resistance. Three common approaches to the study of resistance have been used. In one approach, nucleotide and/or amino acid sequences of virus strains that overcome disease resistance genes in the host are compared with sequences of strains that do not induce disease in these hosts. In the second approach, resistance/susceptibility of protoplasts is compared with the response of intact plants from which they are derived, to develop hypotheses regarding whether resistance acts at the level of the individual cell or by inhibiting cell-to-cell movement. In the third approach, the mechanism of virus cell-to-cell movement has been studied to clarify one of the basic steps in pathogenesis and to determine the mechanism of disease resistance for certain virus-host interactions.