I review recent results on the composition and structure of core-collape supernovae. Models have now been developed from the extreme ends at low mass (~8 Msun) to high-mass (pair instability) that together improve diagnostics of classes from subluminous Type IIP to superluminous Type Ic SNe. The study of emission lines in the nebular phase tells us information about the progenitor star and its nucleosynthesis, as well as the way it exploded.