  The calculation of exclusive observables beyond the one-loop level requires
elaborate techniques for the computation of multi-leg two-loop integrals. We
discuss how the large number of different integrals appearing in actual
two-loop calculations can be reduced to a small number of master integrals. An
efficient method to compute these master integrals is to derive and solve
differential equations in the external invariants for them. As an application
of the differential equation method, we compute the ${\cal O}(\epsilon)$-term
of a particular combination of on-shell massless planar double box integrals,
which appears in the tensor reduction of $2 \to 2$ scattering amplitudes at two
loops.
