Compactification, Geometry and Duality:. N = 2

We review the geometry of the moduli space of N = 2 theories in four dimensions from the point of view of superstring compactification. The cases of a type IIA or type IIB string compactified on a Calabi-Yau threefold and the heterotic string compactified on K3 × T² are each considered in detail. We pay specific attention to the differences between N = 2 theories and N > 2 theories. The moduli spaces of vector multiplets and the moduli spaces of hypermultiplets are reviewed. In the case of hypermultiplets this review is limited by the poor state of our current understanding. Some peculiarities such as "mixed instantons" and the non-existence of a universal hypermultiplet are discussed.