This chapter describes the practical value for attempting to detect and describes the double-stranded (dsRNAs) lies in the ease with which they can be isolated and characterized. Most diagnostic laboratories recognize the value of less specific tests which can be used in addition to specific tests in the early testing stages, especially when the problems are new or unfamiliar. Many of the references cited were selected because they were published since the earlier reviews were written. Selection of tissue may not be particularly critical, since dsRNA is often as abundant in leaves that have been infected for long periods as it is in young new flush tissue. In order to begin dsRNA purification it is first necessary to obtain a total nucleic acid extract from the intended tissue source. The simplest type of analysis is gel electrophoresis of the concentrated dsRNA, followed by staining with ethidium bromide and photography or silver staining.