The concept of time reversal (TR) of scalar wave is reexamined
from basic principles. Five different time reversal
mirrors (TRM) are introduced and their relations are analyzed.

The asymptotic analysis of the near-field focusing property is
presented. It is shown that to have a subwavelength focal spot
the TRM should involve dipole fields. The monopole TR is
extremely ineffective to focus below wavelength as the focal
spot size decreases logarithmically with the distance between
the source and TRM.

Contrary to the matched field processing and the phase processor,
both of which resemble TR, TR in a weak- or non-scattering medium
is usually biased in the longitudinal direction. This is true for
all five TR schemes. On the other hand, the TR focal spot has
been shown repeatedly in the literature, both theoretically and
experimentally, to be centered at the source point when the
medium is multiply scattering. A reconciliation of the two
seemingly conflicting results is found in the random fluctuations
in the intensity of the Green function for a multiply scattering
medium and the notion of scattering-enlarged effective aperture.