  We present the main features of the spin-orbital superexchange which
describes the magnetic and optical properties of Mott insulators with orbital
degrees of freedom. In contrast to the SU(2) symmetry of spin superexchange,
the orbital part of the superexchange obeys the lower cubic symmetry of the
lattice and is intrinsically frustrated. This intrinsic frustration and
spin-orbital entanglement induce enhanced quantum fluctuations, and we point
out a few situations where this leads to disordered states. Strong coupling
between the spin and orbital degrees of freedom is discussed on the example of
the $R$VO$_3$ perovskites, with $R$ standing for rare-earth ion, La,...,Lu. We
explain the observed evolution of the orbital $T_{\rm OO}$ and N\'eel $T_{N1}$
transition temperature in the $R$VO$_3$ series with decreasing ionic radius
$r_R$. A few open problems and the current directions of research in the field
of spin-orbital physics are pointed out.

Strongly Correlated Electrons