  We review the geometrical framework required for understanding the moduli
space of $(2,2)$ superconformal-field theories, highlighting various aspects of
its phase structure. In particular, we indicate the types of phase diagrams
that emerge for ``generic'' Calabi-Yau theories and review an efficient method
for their determination. We then focus on some special types of phase diagrams
that have bearing on the issues of rigid manifolds, mirror symmetry and
geometrical duality.
