  Interstellar dust grains exhibit complex dynamics which is essential for
understanding many key interstellar processes that involve dust, including
grain alignment, grain growth, grain shattering etc. Grain rotational and
translational motions are affected not only by gaseous collisions, but also by
interactions with ions, photons, magnetic fields etc. Some of those
interactions, e.g. interactions of ions with the dipole electric moment of dust
grains, require the quantum nature of the process to be accounted for.
Similarly, coupling of rotational and vibrational degrees of freedom in a grain
happens due to relaxation processes, among which the process related to nuclear
spins frequently is the dominant one. This coupling modifies substantially both
the dynamics of rotational and translational motions by inducing grain flips.
The flipping averages our certain systematic torques and forces that act on
grains. As the rate of flipping is larger for smaller grains, these grains can
be ``thermally trapped'', i.e. rotate at a thermal rate in spite of the
presence of systematic torques. Moreover, a subset of small grains with high
dipole moments may rotate at subthermal rates due to high damping arising from
grain emission in microwave range of frequencies. Translational dynamics of
grains is mostly dominated by grain interactions with magnetohydrodynamic
turbulence. Efficient turbulent mixing of dust grains limits the degree to
which grains of different sizes may be segregated in space.
