The impact of grain boundary junctions on the coarsening of grain boundary networks is reviewed. The various kinds of junctions are introduced, the dynamic steady state configurations are defined, and their equation of motion is derived. It is shown that a limited junction mobility can effectively hinder grain growth, in particular in fine grained materials. The theory is substantiated by computer simulations and supported by experimental results. We propose to utilize Grain Boundary Junction Engineering as an effective tool for microstructure control.