  The thermodynamics of layered antiferro- and ferromagnets with a weak
interlayer coupling and/or easy-axis anisotropy is investigated. A crossover
from an isotropic 2D-like to 3D Heisenberg (or 2D Ising) regime is discussed
within the renormalization group (RG) analysis. Analytical results for the the
(sublattice) magnetization and the ordering temperature are derived are
obtained in different regimes. Numerical calculations on the base of the
equations obtained yield a good agreement with experimental data on La2CuO4 and
layered perovskites. Corresponding results for the Kosterlitz-Thouless and
Curie (N\'eel) temperatures in the case of the easy-plane anisotropy are
derived.
