This chapter reviews the use of standard transmission electron microscopy (TEM) of plastic-embedded material, as well as protocols for the immunolabeling of cryosections, in the analysis of viral interactions with cells. It focuses particularly on the assembly of two types of enveloped viruses: (1) the beta herpesvirus—human cytomegalovirus (HCMV), and (2) the primate lentiviruses—the simian and human immunodeficiency viruses (SIV and HIV). The chapter discusses the ways EM is used to identify morphological features of the various stages in the assembly of virus particles, to distinguish immature and mature particles, or to analyze steps involved in the acquisition of lipid membranes by enveloped viruses. In addition, it demonstrates the way immunolabeling allows the quantification of viral components, even in individual virus particles, and comparisons between particles at different locations in the cell or at different stages in viral assembly. Together with the newly developed methods for electron tomography and correlative immunofluorescence studies and EM, huge potential exists to unravel more details about virus assembly in the near future.