This chapter attempts to review electron microscope methods used in plant virus diagnostics, with reference to in vitro preparations, as opposed to thin sections. In many situations, electron microscopy (EM) is sufficient to solve simple problems, and is also very useful in the phase of “breaking in” a less direct technique such as enzyme-linked immunosorbent assay, or in establishing the precise specificity of a monoclonal antibody. Immunosorbent EM, also sometimes called solid-phase immunoelectron microscopy, is the procedure in which an EM support film is first coated with a layer of antibody, which then serves to trap the virus particles in a subsequently applied virus preparation. In crude virus preparations incubated directly on support films, virus particles are in competition with host materials for a firm anchorage on the grid, and this reduces the number of virus particles retained, while giving a “dirty” background. Sample preparation is very important for successful detection and recognition of plant viruses in fresh material.