Electron tomography in biology is a technique to reconstruct the three-dimensional (3D) structure of macromolecular, subcellular, or cellular objects by transmission electron microscopy (TEM). In this technique, micrographs of the sample are recorded from various orientations by tilting the specimen, which are then computationally merged into a 3D structure. Recent developments in electron tomography, especially in cryo-electron tomography, have allowed us to visualize detailed 3D structures of biological macromolecules in vivo at 2-6. nm resolution, filling the gap between X-ray crystallography or NMR and light microscopy.