Recently developed techniques to measure the structure of interfacial networks in three dimensions have the potential to revolutionize our ability to control the microstructures of polycrystals and interface-dominated materials properties. This paper reviews recent findings from two- and three-dimensional orientation mapping studies. The observations confirm a strong inverse correlation between the relative energies of grain boundaries and the frequency with which they occur in microstructures. The observations also show that during microstructure evolution, relatively higher energy grain boundaries are more likely to be shrinking while lower energy interfaces are more likely to be growing. These processes can lead to a steady-state distribution of grain boundaries that is influenced as much by the relative grain-boundary energies as by the exact processing conditions. Recent findings and emerging opportunities for grain-boundary characterization are reviewed in the final section of the paper.