For a better understanding of supernova explosions the contributors to this volume provide researchers and graduate students in astrophysics with a broad spectrum of alternatives. The confrontation of different theories in one volume should prompt further exploration of the driving piston for the explosions and deeper understanding of the experimental data. Properties of supernova shells are discussed, such as their kinematics, ages, sizes, temperatures, spectra, polarizations, energetics and morphologies. Special attention is given to a few shells of extreme age, viz. G 70.68+1.20, Kepler's SN, and CTB 80, as well as to their statistics.