  The behaviour of uniform elastically isotropic compressible systems in
critical and tricritical points is described in field-theoretical terms.
Renormalizationgroup equations are analyzed for the case of three-dimensional
systems in a two-loop approximation. Fixed points corresponding to various
types of critical and multicritical behaviour under various macroscopic
conditions imposed on the system are distinguished. It is shown that the effect
of the elastic deformations on the critical behaviour of compressible systems
is significant. It manifests itself both in a change in the critical exponents
of Ising magnetic and in the appearance of multicritical points in phase
diagrams at any dimension of the order parameter. It is also shown that, in a
number of experimental investigations, the multicritical behaviour is not
tricritical, as it has been stated, but tetracritical. The influence exerted by
elastic deformations on systems with phase diagrams already containing
multicritical points is analyzed. It is shown that the effect of elastic
deformations leads to a change from bicritical behaviour to a tetracritical
one.
