Progression of Si-based device processing ever deeper into nanoscale dimensions is creating stiffer challenges for the control and manipulation of atomic-scale defects. For example, the increasing use of multiple chemical elements creates a much more complicated network of defect reactions. The growing surface-tovolume ratios that stem from device shrinkage accentuate the importance of defect interactions with surfaces and interfaces, even though the understanding of these interactions remains in embryonic stages. And previously unrecognized effects of temperature on defect creation during ion implantation, as well as of photostimulated defect diffusion during annealing, manifest themselves more clearly in the nanoscale regime. The present work briefly describes these phenomena, and makes the case that modeling of defect behavior through a combination of quantum calculations, molecular dynamics, Monte Carlo, and continuum simulations -?coupled with appropriate experiments -?has become ever more integral to effective defect engineering.