We present an analytical and numerical study of the statistical behaviour of grain boundaries in vortex matter. Dislocation assemblies such as low angle grain boundaries are often observed in this system, determining the emergence of a polycrystalline phase. A theory of vortex polycrystals is proposed, in the conceptual framework of grain boundary pinning. Several aspects, including grain growth, transport properties and hysteretic behaviour are investigated. Results are found in agreement with numerical simulations and experimental observations.