  We present a review of recent works on clustering phenomena in unstable
nuclei studied by antisymmetrized molecular dynamics (AMD). The AMD studies in
these decades have uncovered novel types of clustering phenomena brought about
by the excess neutrons. Among them, this review focuses on the molecule-like
structure of unstable nuclei. One of the earliest discussions on the clustering
in unstable nuclei was made for neutron-rich Be and B isotopes. AMD
calculations predicted that the ground state clustering is enhanced or reduced
depending on the number of excess neutrons. Today, the experiments are
confirming this prediction as the change of the proton radii. Behind this
enhancement and reduction of the clustering, there are underlying shell effects
called molecular- and atomic-orbits. These orbits form covalent and ionic
bonding of the clusters analogous to the atomic molecules. It was found that
this "molecular-orbit picture" reasonably explains the low-lying spectra of Be
isotopes. The molecular-orbit picture is extended to other systems having
parity asymmetric cluster cores and to the three cluster systems. O and Ne
isotopes are the candidates of the former, while the $3\alpha$ linear chains in
C isotopes are the latter. For both subjects, many intensive studies are now in
progress. We also pay a special attention to the observables which are the
fingerprint of the clustering. In particular, we focus on the monopole and
dipole transitions which are recently regarded as good probe for the
clustering. We discuss how they have and will reveal the exotic clustering.
