This is a project in pure mathematics, taking its inspiration from structures in theoretical physics. Quantum field theories have been studied by theoretical physicists for over half a century and have revolutionised the subject. Predictions of quantum field theory have also had a major effect on many areas of pure mathematics over the last twenty years. Unfortunately there is currently no conceptual mathematical understanding of quantum field theory, and the precise calculations which theoretical physicists are able to do have no rigorous basis in pure mathematics. The general aim of this project is to increase our understanding of the mathematics behind quantum field theory. We intend to do this by studying the mysterious relationships between three rigorously defined mathematical objects, each of which was inspired by constructions from theoretical physics. These objects are called stability conditions on triangulated categories, cluster varieties, and Donaldson-Thomas invariants. The key idea to relate them, is something called a Riemann-Hilbert problem.