Abstract: Low frequency gravitational waves from coalescing binary black holes are one of the major science drivers for the space-based gravitational-wave antenna LISA. Such binaries form naturally as a side effect of the hierarchical growth of structure that appears to best describe the evolution of galaxies and galaxy clusters in our universe. The rate at which these coalescences occur is thus likely to be fairly high perhaps several dozen per year especially for events at moderate to high redshift, when structures were merging quite vigorously. In this talk, I will discuss what LISA observations of these mergers can teach us about the cosmic evolution of black holes and structure. These observations will make it possible to measure with exquisite accuracy the coevolution of black hole mass and spin over cosmic time, and perhaps even to help pin down the universe's large geometry.