Spin gap in low-dimensional magnets

The basic mechanisms of spin gap formation in low-dimensional metaloxide compounds are reviewed. Among the objects which show the singlet ground state are dimers, half-integer spin chains, integer spin uniform chains, spin ladders and two-dimensional arrays of exchange coupled spins. In some of these systems the spin gap in magnetic excitation spectrum is inherent, while in others it is formed due to magnetoelastic interaction, charge or orbital ordering.