  The problem of diluted two-dimensional (2D) quantum antiferromagnet (AF) on a
square lattice is studied using spin-wave theory. The influence of impurities
on static and dynamic properties is investigated and a good agreement with
experiments and Monte Carlo (MC) data is found. The hydrodynamic description of
spin-waves breaks down at characteristic wavelengths
$\Lambda\agt\exp(\frac{const}{x})$, $x$ being an impurity concentration, while
the order parameter is free from anomalies. We argue that this dichotomy
originates from strong scattering of the low-energy excitations in 2D.
