Reggeon calculus as a low-order perturbation theory for the Pomeron

We review the foundations of the Gribov Reggeon calculus with an emphasis on the relationship between the energy-plane and J-plane descriptions of the diagrams of the calculus. The question of the "large-rapidity-gap cutoff' for the Pomeron and the problem of signature are treated in more detail than in the traditional approach to the calculus. Except for some slight differences, the main results agree with Gribov's original formulation. We advocate the use of the Reggeon calculus as a refinement on the contemporary "two-component" model for the Pomeron and collect some formulas useful for phenomenological applications.