LOW-ENERGY PHOTON-PHOTON COLLISIONS TO 2-LOOP ORDER

We evaluate the amplitude for gammagamma --> pi0pi0 to two loops in ch iral perturbation theory. The three new counterterms which enter at th is order in the low-energy expansion are estimated with resonance satu ration. We find that the cross section agrees rather well with the ava ilable data and with dispersion theoretic calculations even substantia lly above threshold. Numerical results for the Compton cross section a nd for the neutral pion polarizabilities are also given to two-loop ac curacy.