The phenomenon of cross protection has been known for a long time and in theory it provides an efficient means of controlling virus diseases.  McKinney (1929) was the first to show that plants already infected with a virus are normally protected against infection by a related strain of the virus.  Thus, plants infected with a mild strain could be protected against infection by a related strain of that virus.  However, cross protection referred to here as classical protection has been used to control virus diseases of a few crops.  There are a number of reasons for the limited use and application of classical protection.  First, the fact that a 'live' virus is used to deliberately infect plants makes researchers and growers reluctant to use this approach on a practical scale.  Second, it is generally difficult to obtain mild strains that are of practical value.  Third, there is some possibility that the mild strain might mutate and cause even greater problems than the control strain itself.  Engineered protection, which is a form of parasite-derived resistance, is being pursued in many laboratories as a strategy for controlling virus diseases.  Numerous reports have shown that this approach is generally applicable to many plant viruses without the drawbacks of classical protection.  This report will focus on the analysis of cases where these approaches have been used to manage viruses under field conditions.