  Semidefinite programs are convex optimisation problems involving a linear
objective function and a domain of positive semidefinite matrices. Over the
last two decades, they have become an indispensable tool in quantum information
science. Many otherwise intractable fundamental and applied problems can be
successfully approached by means of relaxation to a semidefinite program. Here,
we review such methodology in the context of quantum correlations. We discuss
how the core idea of semidefinite relaxations can be adapted for a variety of
research topics in quantum correlations, including nonlocality, quantum
communication, quantum networks, entanglement, and quantum cryptography.
