Diseases due to plant viruses cause losses estimated at about $15 billion per year worldwide in both agricultural and horticultural crops. The internationally approved classification and nomenclature of viruses is given by R. I. B. Francki et al. This publication forms a useful companion for those involved in diagnosis. A knowledge of the size, shape, and overall properties of the virus particle are usually important for placing a virus in its correct family or group. Two kinds of techniques of wide applicability in virus diagnosis have emerged. These are serological procedures, especially various forms of enzyme-linked immunosorbent assay and nucleic acid hybridization. Besides the size and morphology of the virus particle as revealed by electron microscopy, various other physical properties of the virus particle have in the past been used in diagnosis. The chapter also presents an overview of the key concepts discussed in this book.