  Direct imaging methods recover the presence, position, and shape of the
unknown obstacles in time-harmonic inverse scattering without a priori
knowledge of either the physical properties or the number of disconnected
components of the scatterer, i.e., on the boundary condition. However, most of
these methods require multi-static data and only obtain partial information
about the obstacle. These qualitative methods are based on constructing
indicator functions defined on the domain of interest, which help determine
whether a spatial point or point source lies inside or outside the scatterer.
This paper explains the main themes of each of these methods, with emphasis on
highlighting the advantages and limitations of each scheme. Additionally, we
will classify each method and describe how some of these methods are closely
related to each other.
