  The production of the radioactive isotopes $^{44}$Ti and $^{60}$Co in all
types of supernovae is examined and compared to observational constraints
including Galactic $\gamma$--ray surveys, measurements of the diffuse 511 keV
radiation, $\gamma$--ray observations of Cas A, the late time light curve of SN
1987A, and isotopic anomalies found in silicon carbide grains in meteorites.
The (revised) line flux from $^{44}$Ti decay in the Cas A supernova remnant
reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper
bound expected from our models. The necessary concurrent ejection of $^{56}$Ni
would also imply that Cas A was a brighter supernova than previously thought
unless extinction in the intervening matter was very large. Thus, if confirmed,
the reported amount of $^{44}$Ti in Cas A provides very interesting constraints
on both the supernova environment and its mechanism. The abundances of
$^{44}$Ti and $^{60}$Co ejected by Type II supernovae are such that
gamma-radiation from $^{44}$Ti decay SN 1987A could be detected by a future
generation of gamma-ray telescopes and that the decay of $^{60}$Co might
provide an interesting contribution to the late time light curve of SN 1987A
and other Type II supernovae. To produce the solar $^{44}$Ca abundance and
satisfy all the observational constraints, nature may prefer at least the
occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia
supernovae. Depending on the escape fraction of positrons due to $^{56}$Co made
in all kinds of Type Ia supernovae, a significant fraction of the steady state
diffuse 511 keV emission may arise from the annihilation of positrons produced
during the decay of $^{44}$Ti to $^{44}$Ca. The Ca and Ti isotopic anomalies in
pre-solar grains confirm the production of $^{44}$Ti in supernovae and that

Astrophysics