  Given two Calabi--Yau threefolds which are believed to constitute a mirror
pair, there are very precise predictions about the enumerative geometry of
rational curves on one of the manifolds which can be made by performing
calculations on the other. We review the mechanics of making these predictions,
including a discussion of two conjectures which specify how the elusive
``constants of integration'' in the mirror map should be fixed. Such
predictions can be useful for checking whether or not various conjectural
constructions of mirror manifolds are producing reasonable answers.
