Electron tomography in biology is a technique to reconstruct the three-dimensional structure of macro-molecular, sub-cellular 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. Combined with recent developments of cryo-electron microscopy, cryo-electron tomography (cryo-ET) have allowed us to visualize detailed 3D structures of biological macromolecules in vivo at sub-atomic resolution, filling the gap between X-ray crystallography or NMR and light microscopy. In this section, we overview this technique and application to biology and discuss future challenge.