Uniqueness method: Multiloop calculations in QCD

The method of uniqueness is extended to diagrams with an arbitrary number of derivatives (or momenta) on the lines. A number of useful formulas is obtained which can be used for calculations in gauge theories. Some examples of multiloop calculations in QCD are given. The method is applied to the calculation of $\alpha_s$-correction to the longitudinal structure function of deep-inelastic lepton-hadron scattering.