  Modern nanomaterials contain complexity that spans all three dimensions -
from multigate semiconductors to clean energy nanocatalysts to complex block
copolymers. For nanoscale characterization, it has been a long-standing goal to
observe and quantify the three-dimensional (3D) structure - not just surfaces,
but the entire internal volume and the chemical arrangement. Electron
tomography estimates the complete 3D structure of nanomaterials from a series
of two-dimensional projections taken across many viewing angles. Since its
first introduction in 1968, electron tomography has progressed substantially in
resolution, dose, and chemical sensitivity. In particular, scanning
transmission electron microscope tomography has greatly enhanced the study of
3D nanomaterials by providing quantifiable internal morphology and
spectroscopic detection of elements. Combined with recent innovations in
computational reconstruction algorithms and 3D visualization tools, scientists
can interactively dissect volumetric representations and extract meaningful
statistics of specimens. This article highlights the maturing field of electron
tomography and the widening scientific applications that utilize 3D structural,
chemical, and functional imaging at the nanometer and subnanometer length
scales.
