  Since a black hole does not emit light from its interior, nor does it have a
surface on which light from nearby sources can be reflected, observational
study of black hole physics requires observing the gravitational impact of the
black hole on its surroundings. A massive black hole leaves a dynamical imprint
on stars and gas close by. Gas in the immediate vicinity of an accreting
massive black hole can, due to the presence of the black hole, shine so
brightly that it outshines the light of the billions of stars in its host
galaxy and be detected across the Universe. By observing the emission from
stars and gas and determining their kinematics scientists can extract vital
information not only on the fundamental properties of the black holes
themselves but also the impact they have on their surroundings. As it turns
out, supermassive black holes appear to play a vital role in shaping the
Universe as we know it, as they can profoundly impact the star formation
history in galaxies. As a consequence, these black holes indirectly impact the
cosmic build up of chemical elements heavier than Helium and thus affect when
and where life can form. For these reasons alone, observations of massive black
holes constitute a very active research area of modern astrophysics.
  In this chapter we aim to provide a general overview -- fit for a non-expert
-- of what scientists have learned, and hope to learn, from analyzing
observations of massive black holes and the material around them.
