  In the last ten years, observational evidences about a possible connection
between Type Ia Supernovae (SNe) properties and the environment where they
explode have been steadily growing. In this paper I discuss, from a theoretical
point of view but with an observer's perspective, the usage of light echoes
(LEs) to probe the CSM around SNe of Type Ia since, in principle, they give us
a unique opportunity of getting a three-dimensional description of the SN
environment. In turn, this can be used to check the often suggested association
of some Ia's with dusty/star forming regions, which would point to a young
population for the progenitors. After giving a brief introduction to the LE
phenomenon in single scattering approximation, I derive analytical and
numerical solutions for the optical light and colour curves for a few simple
dust geometries. A fully 3D multiple scattering treatment has also been
implemented in a Monte Carlo code, which I have used to investigate the effects
of multiple scattering. In particular, I have explored in detail the LE colour
dependency from time and dust distribution, since this is a promising tool to
determine the dust density and derive the effective presence of multiple
scattering from the observed properties. Finally, again by means of Monte Carlo
simulations, I have studied the effects of multiple scattering on the LE linear
polarization, analyzing the dependencies from the dust parameters and geometry.
Both the analytical formalism and MC codes described in this paper can be used
for any LE for which the light curve of the central source is known.
