  We review the theory of second--order (ferro--)elastic phase transitions,
where the order parameter consists of a certain linear combination of strain
tensor components, and the accompanying soft mode is an acoustic phonon. In
three--dimensional crystals, the softening can occur in one-- or
two--dimensional soft sectors. The ensuing anisotropy reduces the effect of
fluctuations, rendering the critical behaviour of these systems classical for a
one--dimensional soft sector, and classical with logarithmic corrections in
case of a two--dimensional soft sector. The dynamical critical exponent is $z =
2$, and as a consequence the sound velocity vanishes as $c_s \propto | T - T_c
|^{1/2}$, while the phonon damping coefficient is essentially
temperature--independent. Disorder may lead to a variety of precursor effects
and modified critical behaviour. Defects that locally soften the crystal may
induce the phenomenon of local order parameter condensation. When the
correlation length of the pure system exceeds the average defect separation
$n_{\rm D}^{-1/3}$, a disorder--induced phase transition to a state with
non--zero average order parameter can occur at a temperature $T_c(n_{\rm D})$
well above the transition temperature $T_c^0$ of the pure crystal. Near
$T_c^0$, the order--parameter curve, susceptibility, and specific heat appear
rounded. For $T < T_c(n_{\rm D})$ the spatial inhomogeneity induces a static
central peak with finite $q$ width in the scattering cross section, accompanied
by a dynamical component that is confined to the very vicinity of the
disorder--induced phase transition.
