  Half BPS correlators in N=4 super Yang-Mills theory are key quantities both
in the AdS/CFT correspondence as well as in scattering amplitudes research.
They are dual at strong coupling to quantum gravity amplitudes. At weak
coupling on the other hand they contain all N=4 SYM amplitudes. They have been
found to possess a number of hidden symmetries, for example non-trivial
permutation symmetry of perturbative four-point integrands and a higher
dimensional conformal symmetry. Their study has enjoyed continuous progress
since the discovery of AdS/CFT and they are now some of the best understood
quantities of any four-dimensional quantum field theory.
  In this review we outline the current knowledge of half BPS correlators,
emphasising these two co-existing relations to scattering amplitudes at strong
and weak coupling.
