Dr. Peter Ercius provides a tutorial on 3D Electron Tomography. Transmission electron microscopy (TEM) provides sub-Angstrom image resolution such that the atomic structure of materials is readily visible. Such high-resolution TEM images are routinely used to analyze nano-structures in materials science. However, these are only projections of complex three-dimensional structures, and the lost information is critical to determine structure/function relationships. Electron tomography is a technique that recovers the lost information along the projection direction from a series of 2D images at different viewing angles and has become increasingly important for quantitative 3D characterization of a wide range of materials. This tutorial will cover the basic theory, image processing and reconstruction algorithms of electron tomography and recent advancements in atomic resolution tomography.