Supernovae represent the last stage of stellar evolution, are responsible for
the majority of the heavy elements in the universe, are sites of
particle acceleration, and are the birth places for neutron stars and
black holes. At Stockholm Observatory the nucleosynthesis is studied from observations at
stages later than about 100 days after explosion, when the expansion
makes it possible to study the interior of the ejecta. The degradation of
the 
-rays from the radioactive decay of nickel and cobalt to
optical and IR photons is modelled in order to compare with
observations in these wavelength ranges. Predictions for the
appearance of the new-born, not yet observed, neutron star are
made. Ground-based and HST
observations of especially the supernova SN 1987A are used for the comparison with
the models.
Another important area is the interaction of supernovae with the
circumstellar medium of the
progenitor star. This interaction dominates in many cases the
observational appearance of the supernova, giving crucial information
about the nature of the progenitor of the supernova. It is also a unique
laboratory for many processes associated with shock waves, of general
importance for high energy
astrophysics. Observations in radio, optical, UV, X-rays and
-rays are modelled in order to understand both the thermal
emission from the shock and ejecta, as well as non-thermal
processes. In particular, the radio emission offers the possibility to
study the acceleration of the relativistic electrons in a well
understood environment and geometry, which is of great interest for the
understanding of cosmic ray acceleration. Objects of special interest
are the ring of SN 1987A and SN 1993J in M81.