Recent improvements in electron microscope techniques which allow the study of virus fine structure have permitted the grouping of many viruses on a purely morphological basis. Briefly the techniques used in electron microscopy for the study of viruses are reviewed and the symmetry properties of virus particles as revealed by negative staining are discussed somewhat more fully.Finally, virus particles are grouped on two bases, firstly the site of formation of the virus within the cell as seen by thin sectioning techniques, and secondly the symmetry property of the virus as seen by negative staining. Consideration of the groupings obtained in this way reveals that the biochemical and physical properties of a virus can be deduced from the readily established morphological characteristics.