  We give an overview of complementarity and synergy in cosmology between the
Square Kilometre Array and future survey projects in other wavelengths. In the
SKA era, precision cosmology will be limited by systematic errors and cosmic
variance, rather than statistical errors. However, combining and/or
cross-correlating multi-wavelength data, from the SKA to the cosmic microwave
background, optical/infrared and X-ray, substantially reduce these limiting
factors. In this chapter, we summarize future survey projects and show
highlights of complementarity and synergy, which can be very powerful to probe
major cosmological problems such as dark energy, modified gravity and
primordial non-Gaussianity.
