2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/149747The point is to compare the mathematical meaning of the ``number of rational curves on a Calabi-Yau threefold'' to the meaning ascribed to the same notion by string theorists.10 pages, AMSLaTeX v1.0 or 1.1. This is a survey talk given at the May 1991 Workshop on Mirror Symmetry at MSRIAlgebraic Geometrytext