The evolution of grain size and shape as well as type and frequency of grain boundary structures during thermal annealing of lamellar diblock copolymer microstructures is established using large area image reconstruction and analysis. Grain coarsening is found to proceed via an initial transient stage that is characterized by the rapid relaxation of unstable "frozen-in" defects such as kink boundaries and the subsequent quasi-stationary coarsening that is dominated by the continuous relaxation of low-angle symmetric tilt boundaries. The particular relevance of low-angle symmetric tilt boundaries to grain coarsening is interpreted as the consequence of both the associated decrease of boundary energy as well as the availability of favorable kinetic pathways-such as grain boundary splitting-to facilitate the coarsening process. The inverse relation between grain boundary energy and frequency suggests that the reduction of boundary energy is a relevant governing parameter for the evolution of grain boundary structures-as it is in inorganic materials. The existence of "inert" boundary types (such as asymmetric tilt and twist) that-within the experimental window-do not participate in the coarsening process is expected to have dominant influence on the final morphology that can be attained by thermal annealing of the microstructure. The reduction of the density of inert boundaries during the film preparation process should therefore provide a strategy for increasing the coarsening kinetics in block copolymer films during thermal annealing and thus a path toward a higher degree of order in block copolymer microstructures.