After a short introduction on how S-matrix theory and perturbative QCD (through DGLAP evolution) can be used in Deep Inelastic Scattering (DIS), I shall develop new approaches in these domains. Firstly, I shall show how we can use t-channel unitarity to obtain the photon-photon amplitude from the proton-proton and the photon-proton amplitudes and how it can be applied to multiple-pole pomerons. Then, I shall explain how Regge theory can constrain the initial parton distributions in QCD global fits. In this framework, I shall discuss the applicability of Regge theory at small and large Q2 and its consequences.

Université de Liège, Belgique