  A unified theory of phase transitions and quantum effects in quantum
anharmonic crystals is presented. In its framework, the relationship between
these two phenomena is analyzed. The theory is based on the representation of
the model Gibbs states in terms of path measures (Euclidean Gibbs measures). It
covers the case of crystals without translation invariance, as well as the case
of asymmetric anharmonic potentials. The results obtained are compared with
those known in the literature.
