  Measurements of SN rates in different environments and redshifts can shed
light on the nature of SN-Ia progenitors, star formation history, and chemical
enrichment history. I summarize some recent work by our group in this area, and
discuss the implications. The current evidence favors production of most of the
iron in the ICM (and perhaps everywhere) by core-collapse SNe, rather than
SNe-Ia. These SNe may have been produced by the first, top-heavy-IMF,
generation of stars that reionized the Universe. Improved rate measurements can
sharpen the picture, and I describe our recent efforts in this direction.

Astrophysics