  An increasing variety of crystal structures has been observed in soft
condensed matter over the past two decades, surpassing most expectations for
the diversity of arrangements accessible through classical driving forces.
Here, we survey the structural breadth of mesoscopic crystals -- formed by
micellar systems, nanoparticles, colloids etc. -- that have been observed both
in soft matter experiments and coarse-grained self-assembly simulations. We
review structure types that were found to mimic crystals on the atomic scale,
as well as those that do not correspond to known geometries and seem to only
occur on the mesoscale. While the number of crystal structure types observed in
soft condensed matter still lags behind what is known from hard condensed
matter, we hypothesize that the high tunability and diversity of building
blocks that can be created on the nano- and microscale will render a structural
variety that far exceeds that of atomic compounds, which are inevitably
restricted by the "limitations" imposed by the periodic table of elements and
by the properties of the chemical bond. An infusion of expertise in structural
analysis from the field of crystallography into the soft condensed matter
community will establish the common language necessary to report, compare, and
organize the rapidly accruing structural knowledge gathered from simulations
and experiments. The prospect of new materials created in soft matter and new,
length-scale-spanning insights into the formation of ordered structures in both
hard and soft condensed matter promise exciting new developments in the area of
self-assembled mesoscale materials.
