The aim of this paper is to introduce the theory of Abelian
integrals for holomorphic foliations in a complex manifold of
dimension two. We will show the importance of Picard-Lefschetz
theory and the classification of relatively exact 1-forms in this
theory. As an application we identify some irreducible components
of the space of holomorphic foliations of a fixed degree and with
a center singularity in the projective space of dimension two.
Also we calculate higher Melnikov functions under some generic
conditions.