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.