Abstract
I'll review the construction of topological string theory which has a lot of applications in usual string theory and mathematics. I'll start by summarizing the geometry of Calabi-Yau manifolds, where topological strings will be defined, introduce topological twist, define A and B models, and discuss a relation to F-terms in the effective action of type II strings. In the end, I'll explain the wave function property related to the issue of background independence.