Abstract: Calculations of the number of curves on a Calabi-Yau manifold via an instanton expansion do not always agree with what one would expect naively. It is explained how to account for continuous families of instantons via deformation theory and excess intersection theory. The essential role played by degenerate instantons is also explained. This paper is a slightly expanded version of the author's talk at the June 1995 Trieste Conference on S-Duality and Mirror Symmetry.