The Theory and Simulation Group developes efficient methods for atomistic simulations in nanostructured systems, which can fully exploit modern computer multiprocessor architectures, and applies them to selected problems in Nanoscience and Nanotechnology. These include (but are not restricted to) the SIESTA and TranSIESTA codes. SIESTA is a multi-purpose first-principles method and program, based on Density Functional Theory, which can be used to describe the atomic and electronic properties of systems with up to several thousands of atoms. TranSIESTA is an extension of SIESTA that enables the study of electronic transport phenomena in nanoscale devices. Both codes are among the most important of their kind and are widely used by the academic community.