Abstract. We give a detailed account of the recently formulated generalised vector dominance/colour-dipole picture (GVD/CDP) of deep-inelastic scattering at low , including photoproduction. The approach, based on transitions, propagation and diffractive scattering via the generic structure of the two-gluon exchange, provides a unique and quantitatively successful theory for the total cross section, , at low x. The GVD/CDP is shown to imply the empirical low-x scaling law, with , that was established by a model-independent analysis of the experimental data.