  Exoplanetary science is a very active field of astronomy nowadays, with
questions still opened such as how planetary systems form and evolve
(occurrence, process), why such a diversity of exoplanets is observed (mass,
radius, orbital parameters, temperature, composition), and what are the
interactions between planets, circumstellar disk and their host star. Several
complementary methods are used for the detection of exoplanets. Among these,
imaging aims at the direct detection of the light reflected, scattered or
emitted by exoplanets and circumstellar disks. This allows their spectral and
polarimetric characterization. Such imaging remains challenging because of the
large luminosity ratio (1e4-1e10$) and the small angular separation (fraction
of an arcsecond) between the star and its environment. Over the past two
decades, numerous techniques, including coronagraphy, have been developed to
make exoplanet imaging a reality.
  This review gives a broad overview of the subsystems that make up a
coronagraphic instrument for imaging exoplanetary systems. It is especially
intended for non-specialists or newcomers in the field. We explain the
principle of coronagraphy and propose a formalism to understand their behavior.
We discuss the impact of wavefront aberrations on the performance of
coronagraphs and how they induce stellar speckles in the scientific image.
Finally, we present instrumental and signal processing techniques used for
on-sky minimization or a posteriori calibration of these speckles in order to
improve the performance of coronagraphs.
