  Up to now, Einstein's general theory of relativity has passed all
experimental tests. But yet we know that it is not a fundamental theory and
that it is incompatible with quantum theory. While several extended and
improved gravitational theories on classical and quantum level are nowadays
available, it is a great challenge to find experimental setups to check their
signatures.
  We discuss recent developments in direct observation of black holes
comprising gravitational waves from black hole mergers, radio interferometry
images of black hole shadows, and Hawking radiation of black holes in particle
accelerators. These investigations cover the full black hole mass range from
microscopic to stellar and supermassive black holes. We comment on the
associated strong-field tests of Einstein's general theory of relativity and
implications for quantum gravity.
  Special emphasis lies upon the physics of large extra dimensions and black
hole evaporation, the existence of a minimal black hole mass, and the cross
sections of higher-dimensional black holes. We complete this short review with
the latest experimental constraints at the Large Hadron Collider.
