  The paper is devoted to the mathematical aspects of topological quantum field
theory and its applications to enumerative problems of algebraic geometry. In
particular, it contains an axiomatic treatment of Gromov-Witten classes, and a
discussion of their properties for Fano varieties. Cohomological Field Theories
are defined, and it is proved that tree level theories are determined by their
correlation functions. Applications to counting rational curves on del Pezzo
surfaces and projective spaces are given.
