  In this review we present recent theoretical results concerning
investigations of single subsurface defects by means of a scanning tunneling
microscope (STM). These investigations are based on the effect of quantum
interference between the electron partial waves that are directly transmitted
through the contact and the partial waves scattered by the defect. In
particular, we have shown the possibility imaging the defect position below a
metal surface by means of STM. Different types of subsurface defects have been
discussed: point-like magnetic and non-magnetic defects, magnetic clusters in a
nonmagnetic host metal, and non-magnetic defects in a s-wave superconductor.
The effect of Fermi surface anisotropy has been analyzed. Also, results of
investigations of the effect of a strong magnetic field to the STM conductance
of a tunnel point contact in the presence of a single defect has been
presented.
