Abstract: Multi wavelength observations of the sky from a number of planned and upcoming cosmological surveys will provide unprecedented opportunities to understand the nature of dark energy, dark matter, neutrinos and inflation. With the increased precision, the effects of systematic biases are becoming increasingly important, requiring significant improvements in our understanding of several observational and astrophysical processes. The complementarity of different surveys and the overlapping observations provide new opportunities to exploit the synergies between different probes, to study the fundamental physics and to control the impact of several systematic biases. In my talk, I will show examples of synergies between different probes from our work combining galaxy clustering and weak gravitational lensing measurements to constrain parameters of cosmological models and to perform tests on gravity. I will then discuss the challenges we face in combining information from increasingly larger volume surveys and some of the solutions we are working on. I will also highlight the new opportunities we will have to study and mitigate the effects of systematics with the examples of photometric redshifts of galaxies and the intrinsic alignments of galaxy shapes.