Summary: We will establish a satellite service center within a larger network according to RFAs RM-19-009 and -010, named the CU-Boulder Center for Cryo-Electron Tomography (CCET). Our satellite will focus on cryo- specimen preparation and cryo-electron tomography (cryo-ET) applied to cells, organelles and large supramolecular assemblies. The applicants combine the expertise of three P.I.s, Michael Stowell (high- resolution cryo-electron microscopy (cryo-EM)), Karolin Luger (biochemistry of large complex assemblies, especially protein-nucleic acid interactions), and Andreas Hoenger (former P41 director, cryo-EM and cryo-ET from macromolecules to cells and tissues). With our current staff, we combine extensive expertise with cryo-ET and relevant software. We will offer a broad range of cryo-electron microscopy (cryo-EM) and cryo-ET preparation procedures for cellular structures and intracellular macromolecular assemblies, as well as training opportunities for scientists who would like to establish these technologies, or at least part of them, for their home labs. Here in Boulder, we have several high-end microscopes available, including a newly installed Titan-Krios. Three of our microscopes are equipped with direct electron detectors. In addition, we have a comprehensive periphery for cryo-specimen preparation, storage, and handling resources, including computational tools. We will add cryo-Focused Ion Beam (cryo-FIB) milling to produce vitrified lamellae of cells and tissues. We propose the following organization of the facility: Specific Aim 1: Cryo-ET specimen preparation methods offered for service: Our technology and expertise will be made accessible for our users and for further collaborations with the data collection hub proposed in a parallel RFA (RM-19-010). Specific Aim 2: Cross-training opportunities for service users: We will offer comprehensive training for users, including one-on-one instruction on microscopes and related equipment. As appropriate, we will pool users into small groups and run a training week as a hands-on workshop on equipment as well as related software developed by our facility such as Serial-EM, IMOD and PEET. While the emphasis with training will be on practical aspects of specimen preparation and data collection, we often encounter novice users in strong need for a thorough introduction into image analysis software as well. Specific Aim 3: Administration and organization of the facility: Users, assigned to us via the network-wide application process will have the opportunity to participate in the specimen refinement process, and receive training on the cryo-ET processes involved. Projects with high potential will be referred for high-throughput data collection to the central hub facility of the network. Boulder, Colorado is centrally located and well-connected nationwide through Denver International Airport.