  The extreme luminosity and their fairly unique temporal behaviour have made
supernovae a superb tool to measure distances in the universe. As complex
astrophysical events they provide interesting insights into explosion physics,
explosive nucleosynthesis, hydrodynamics of the explosion and radiation
transport. They are an end product of stellar evolution and provide clues to
the stellar composition. Since they can be observed at large distances they
have become critical probes to further explore astrophysical effects, like dust
properties in external galaxies and the star formation history of galaxies.
Some of the astrophysics interferes with the cosmological applications of
supernovae. The local velocity field, distorted by the gravitational attraction
of the local large scale structure, and the reddening law appear at the moment
the major limitations in the accuracy with which cosmological parameters can be
determined. These absorption effects can introduce a secondary bias into the
observations of the distant supernovae, which needs to be carefully evaluated.
Supernovae have been used for the measurement of the Hubble constant, i.e. the
current expansion rate of the universe, and the accelerated cosmic expansion
directly inferred from the apparent faintness of the distant supernovae.
