The physics program of the ALFA detector in ATLAS will be discussed. The ALFA detector is a scintillating g fiber tracker installed in Roman Pots on both sides of the ATLAS interaction point. After a short introduction to the notion of Roman Pots  and a description of the main feature of the detector, its physics possibilities will be discussed. This will also include some recent results. Although the physics that ALFA covers is low pt physics, out of reach for QCD calculations,  results and potential measurement will be discussed in terms of general principles. Some emphasis will be given to dispersion relations.