Cryo-EM/ET Core Cryo-EM and cryo-ET are powerful techniques for high-resolution structural studies of proteins and protein complexes, complementing X-ray crystallography and NMR methods. Cryo-ET with subtomogram averaging and classification, in particular, has recently gained abundant attention as a prime method for characterization of macromolecular complexes that are intrinsically flexible and often challenging to purify for structure determination by single particle cryo-EM. Moreover, in situ cryo-ET can inform on structures of complexes within their native cellular environment, with near atomic resolution. The Cryo-EM/ET Core will play a key role in developing and applying these cutting-edge methods for structural analysis of heterogeneous HIV-1 viral components and their complexes with host cell factors and small molecule inhibitors. The Core will support PCHPI projects by providing structural information on assembly and maturation intermediates as well as capsid-host interactions during nuclear import and integration. Furthermore, the Core will offer new capabilities of cryo-EM and cryo-ET imaging of infectious samples in a biosafety level 3 (BSL-3) environment and develop new technologies and workflows for single-molecule correlation between cryo-ET and super-resolution fluorescence microscopy. These approaches will yield transformative structural and mechanistic information into the key steps of HIV-1 replication.