  In this paper we study extremal transitions between heterotic string
compactifications, i.e., transitions between pairs (M,V) where M is a
Calabi-Yau manifold and V a gauge bundle. Bundle transitions are described
using language recently espoused by Friedman, Morgan, Witten. In addition,
partly as a check on our methods, we also study how small instantons are
described in the same language, and also describe the sheaves corresponding to
small instantons.
