Diagnosis of plant virus infections has been greatly assisted by the classification of viruses into groups. A number of methods have been developed for the detection and diagnosis of virus diseases. The quality of the plant sample will often determine the choice of diagnostic method, the amount of replication, and the probability of success in problem definition. The viral groups of greatest importance primarily represent those possessing inclusions generally distributed throughout plant tissues. Inclusions are considered as a main characteristic of most recognized plant virus groups. A number of plant viruses induce nuclear inclusions. Selection of plant inclusion methodology offers strength above all other viral diagnostic technologies. An inclusion shown to consist of virus particles with electron microscopy can be similarly identified in the light microscope as a virus aggregate, even though individual particles cannot be resolved by light microscopy.