  A review is given on the studies of formation of light clusters and heavier
fragments in heavy-ion collisions at incident energies from several tens of
MeV/nucleon to several hundred MeV/nucleon, focusing on dynamical aspects and
on microscopic theoretical descriptions. Existing experimental data already
clarify basic characteristics of expanding and fragmenting systems typically in
central collisions, where cluster correlations cannot be ignored. Cluster
correlations appear almost everywhere in excited low-density nuclear many-body
systems and nuclear matter in statistical equilibrium where the properties of a
cluster may be influenced by the medium. On the other hand, transport models to
solve the time evolution have been developed based on the single-nucleon
distribution function. Different types of transport models are reviewed putting
emphasis both on theoretical features and practical performances in the
description of fragmentation. A key concept to distinguish different models is
how to consistently handle single-nucleon motions in the mean field,
fluctuation or branching induced by two-nucleon collisions, and localization of
nucleons to form fragments and clusters. Some transport codes have been
extended to treat light clusters explicitly. Results indicate that cluster
correlations can have strong impacts on global collision dynamics and
correlations between light clusters should also be taken into account.
