  In the recent years, we witness a great interest in imaging, in a wide sense,
using contrast agents. One of the reasons is that many imaging modalities, as
the ones related to medical sciences, suffer from several shortcomings. The
most serious one is the issue of instability. Indeed, it is, nowadays, a common
certainty that classical inverse problems of recovering objects from remote
measurements are, mostly, highly unstable. To recover the stability, it is
advised to create, whenever possible, the missing contrasts in the targets to
image. In this survey paper, we follow this direction and propose an approach
how to analyze mathematically the effect of the injected agents on the
different fields under consideration. These contrast agents are small-sized
particles modeled with materials that enjoy high contrasts as compared to the
ones of the background. These two properties allow them, under critical scales
of size/contrast, to create local spots when excited from far. These local
spots can be remotely recovered in stable ways. The accessible information on
the target are encoded in theses spots.
  After stating a class of such imaging modalities that enter into this
framework, as the acoustic imaging, photo-acoustic imaging, optical imaging and
more, we provide detailed analysis for first two modalities where the contrast
agents are micro-bubbles and nano-particles respectively.
