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The emerging discipline of electron tomography provides new and unprecedented opportunities to determine three-dimensional cellular architecture at resolutions of ~50A or better, i.e., potentially high enough to identify individual macromolecules such as proteins in a 3-D volume of a cell289. Electron tomography is especially applicable for the structural analysis of cellular organelles and other macromolecular assemblies that are too heterogeneous to be investigated by NMR or X-ray crystallographic techniques or even electron microscope-based methods that involve averaging of multiple copies of the same object to improve signal-tonoise ratios. Electron tomography bridges the gap between high resolution structure determination of protein complexes by NMR or X-ray crystallographic techniques and single-particle living cell imaging by light microscopy using fluorescent probes. It extends the resolution of cellular imaging by one to two orders of magnitude over what is currently achieved using light microscopy. The fundamental principles underlying electron tomography and the strategy to use back-projection algorithms to extract 3-D information from a series of 2-D images recorded at different orientations were clearly articulated nearly four decades ago290'291. However, this field has seen a significant burst of activity in the last five years, principally propelled by the availability of tools for automated data acquisition using modern computerized microscopes. Efforts at imaging complex assemblies at room temperature as well as cryogenic temperatures have rapidly begun to provide many new insights into the 3-D architecture of cells289'292. During the immediate post-entry stages in HIV pathogenesis, the viral particle or, more precisely, its contents must traverse through the cytoplasm of the host cell prior to nuclear import of the pre-integration complex (PIC). Identification and characterization of these cellular events and viral and host interactions are central to understanding the molecular events that occur during HIV pathogenesis. NMR or X-ray crystallographic methods will provide high resolution structures of the viral and host protein complexes, while confocal microscopy in combination with fluorescent-labeling techniques allows tracking of viral particles and assessing the approximate localization of individual proteins in living cells. However, advancement of electron microscopic techniques, particularly electron tomography, will be essential to obtain higher resolution snapshots of the molecular arrangement of multiprotein complexes in the context of the host cellular structure and to provide the structural information necessary to fill the gap between NMR or X-ray crystallographic structural determination and live cell imaging. We, therefore, expect that our work to develop approaches for 3- D cellular imaging will have a direct impact on cellular and structural studies of HIV infection.