  We compare model calculations with observations of supernovae at late times
to infer the time evolution of temperature, ionization and line emission. Here
we mainly report on our results from our modeling of SN 1987A. We discuss the
oxygen mass from the modeling of line fluxes. Line profiles show the
distribution of the elements and the importance of including time dependence in
the calculations. We discuss different approaches to determine the 44Ti-mass.
