In this talk we take the first steps towards a new framework for computing two-loop amplitudes, based on unitarity rather than Feynman diagrams. In this approach, the two-loop amplitude is first expanded in a basis of integrals. The expansion coefficients are then determined by applying generalized unitarity cuts. We find explicit formulas for the integral coefficients as products of tree level amplitudes integrated over specific contours in the complex plane, thus allowing to construct the two-loop amplitude from appropriately defined tree amplitudes. The validity of this method extends to all 4-dimensional gauge theories, in particular QCD. This approach is suited for obtaining analytical expressions as well as for numerical implementations.