The development of materials and devices at the nanoscale presents great challenges, from synthesis to assembly to characterization. Often, progress can only be made by complementing work with electronic-structure modeling, harnessing the efficiency, predictive power, and atomic resolution of density functional theory to describe molecular architectures exactly at those scales (hundreds or thousands of atoms) where the most promising and undiscovered properties are to be engineered. Some of the next-genertion technologies that will benefit first from first-principles simulations encompass areas as diverse as energy and information storage and retrieval, detection and sensing of biological and foreing contaminants, nanostructured catalysts, nanomechanical devices, hybrid organic-inorganic and biologically inspired materials, and novel computer technologies based on integrated optical and electronic platforms. This article reviews some of the recent successes and insights gained by electronic-structure modelling, ranging from carbon nanotubes to semiconducting nanoparticles, quantom dots, and self-assembled monolayers.