Science: The often dramatic effects of particle irradiation on the properties of materials have been recognized and studied for over 60 years. These effects can be detrimental (as in structural materials degradation in nuclear reactors) or beneficial (as in the ion beam processing of semiconductors for the microelectronics industry). However, the microscopic processes that underlie these effects are not entirely understood, limiting researchers' ability to predict the consequences of irradiation. The reports in this week's issue of Science highlight the limits of knowledge about nanometer-sized dislocation loops in materials. The results should stimulate additional research to better understand these phenomena and to use the unique diffusion behavior to pattern materials at the nanoscale.