  Double parton scattering events are directly sensitive to the correlations
between two partons inside a proton and can answer fundamental questions on the
connections between the proton constituents. In this chapter, the different
types of possible correlations, our present knowledge of them, and the
processes where they are likely to be important, are introduced and explained.
The increasing integrated luminosity at the LHC and the refinements of the
theory of double parton scattering, lead to interesting prospects for
measuring, or severely constraining, two-parton correlations in the near
future.
