  The recent availability of radioactive beams has opened up a new era in
nuclear physics. The interactions and structure of exotic nuclei close to the
drip lines have been studied extensively world wide, and it has been revealed
that unstable nuclei, having weakly bound nucleons, exhibit characteristic
features such as a halo structure and a soft dipole excitation. We here review
the developments of the physics of unstable nuclei in the past few decades. The
topics discussed in this Chapter include the halo and skin structures, the
Coulomb breakup, the dineutron correlation, the pair transfer reactions, the
two-nucleon radioactivity, the appearance of new magic numbers, and the pygmy
dipole resonances.
