Core-collapse SNe offer unique opportunities to look inside massive stars. By 
calculating the deposition and degradation of gamma-rays and positrons, solving the 
equations for statistical and thermal equilibrium, and considering the effects of multi-
line radiative transfer, we produce model spectra that can be compared with 
observations. We apply our model to interpret the spectrum of SN 1987A at an age 
of 8 and 19 years, finding a powering of ~1.5E-4 Msun of 44Ti in the iron/silicon core 
to satisfactorily reproduce most of the observed lines.  We compare models with local 
and non-local positron deposition, and find better agreement assuming local trapping. 
The cold and neutral ejecta produce strong Fe I lines. We also see that radiative 
transfer effects still play a crucial role for the spectral formation even many years 
after explosion. One outstanding problem is, however, to explain the weak Fe II 26 
mu line observed by Spitzer. I also briefly discuss current work on modeling other 
Type IIP SNe in the 200-600 day range.