The implementation of efficient quantum gates between individual photons is an important goal in quantum information processing. Achieving this goal poses two complementary challenges. On the one hand, it requires strong interactions between individual photons. On the other hand, the gate operations have to be realized in such a way that they preserve the single-mode character of the photons despite those strong interactions. I will give a brief overview over different approaches that have been proposed, and discuss where they stand with respect to these two challenges. In particular I will explain the limitations due to transverse multi-mode effects, and show how they could be overcome in a scheme based on dipole-dipole interactions between Rydberg state polaritons . Finally I will present a recent proposal for photon-photon gates in Bose-Einstein condensates , which exploits the long storage times and collisional interactions available in this system, where the interactions are enhanced by the adiabatic compression of the condensate and the use of a Feshbach resonance. I will conclude by arguing that we may have reached a critical mass of technology and theoretical understanding to allow successful experiments on efficient photon-photon gates in the foreseeable future.
 B. He, A. MacRae, Y. Han, A.I. Lvovsky, and C. Simon, Transverse multi-mode effects on the performance of photon-photon gates, arXiv:1006.3584
 A. Rispe, B. He, and C. Simon, Photon-photon gates in Bose-Einstein condensates, arXiv:1010.0037