Stable nuclei in nature are found within a narrow band of proton and neutron numbers, Z and N respectively. Beyond the domain of these stable nuclei, a large number of nuclei, super-rich in protons for a given N, and super-rich in neutrons for a give Z, have been produced over the last decade or so at several accelerator centres round the world. They are beta active i.e. they decay by positron emission on the high Z side and by electron emission on the high N side and finally end up on the time of stability in the (Z,N) plot of the nuclei. For a given Z a limit to addition of neutrons is reached when the resultant nucleus becomes unstable under particle emission and undergoes a fast decay by neutron emission. The zig-zag line traced by these border-line unstable nuclei on the neutron-rich side is called the neutron-drip line. Similarly, on the proton-rich side one finds a proton-drip line along which the nuclei undergo quick decay by a proton emission. The beta active nuclei on both sides of the region of stability bounded by the two drip lines have measurable half lines, long enough for the study of their systematic properties by state of the art experimental techniques. After a general review of the various experimental features, a brief account will be given of the theoretical work by several groups, including the theory group at IOP, mainly based on the relativistic mean-field formalism. Some of the aspects of the theory, specially related to the treatment of pairing, will be commented upon. My own fairly old work on the two neutron halo will be briefly mentioned.
