Three-dimensionalization, i.e., direct representation or fabrication of three-dimensional (3D) objects, is now a key technology in various science and engineering fields. Electron microscopy, a vital nanoscale characterization tool, is no exception, thus various imaging methods have been developed to extend its imaging capabilities from conventional two dimensions to three dimensions. In this article, we focus on electron tomography (ET), which is a typical 3D imaging method using transmission electron microscopy (TEM) or scanning transmission electron microscopy(STEM), and overview the current status and future prospects of ET and an application of ET to 3D imaging of dislocations in crystalline materials as an practical example.