The Jensen lab at Caltech studies the structure and function of large protein assemblies by electron cryotomography, an emerging structural method for supramolecular complexes. In collaboration with the Sundquist group, they recently generated the first 3-D structures of both mature and immature HIV virions, which revealed the role of the SP1 spacer peptide in the immature Gag lattice and confirmed the fullerene cone model of the mature capsid shell. Together with the Hill and Sundquist groups, they have now solved a low-resolution structure of Vps4, a AAA-ATPase involved in HIV budding, by EM-based single particle analysis. The Jensen group is also advancing electron cryotomography technology through pioneering use of new instrumentation, automation, and image processing innovations.