Abstract
The linear response to an electromagnetic field of a gas of particles in a random on-site potential is studied within the ordinary many-body theory and the multiplicative renormalisation group. The crucial role played by the gauge invariance in determining the renormalisation procedure and thus the critical indices is emphasised by means of the use of the corresponding Ward identities. In this way it is possible to rederive a complete scaling theory of the localisation transition. A gauge-invariant expansion in terms of the inverse conductance around the lower critical dimensionality two leads then to the epsilon -expansion for the critical indices, which was previously derived by referring to an analogy with the non-linear sigma model.