  A recently introduced method for constructing marginal singular flows between
distinct Landau--Ginzburg theories at fixed central charge is reviewed. The
flows are constructed in an enlarged moduli space obtained by adding theories
with zero central charge. This mechanism is used to construct flows between
mirror pairs of string vacua described by N$=$2 superconformal Landau--Ginzburg
fixed points. In contrast to previous methods this new construction of mirror
theories does not depend on particular symmetries of the original theory.
  (Based in part on a talk presented at the Trieste Summer
  Workshop on Superstrings and Related Topics, July 1992)
