SIESTA is a first-principles materials simulation program based on density-functional theory (DFT). It was one of the first codes to enable the treatment of large systems with first-principles electronic-structure methods, which opened up new research avenues in many disciplines. SIESTA has become quite popular and is increasingly being used by researchers in geosciences, biology and engineering (apart from those in its natural habitats of materials physics and chemistry). SIESTA‘s efficiency stems from the use of strictly localized basis sets and from the implementation of linear-scaling algorithms which can be applied to suitable systems. A very important feature of the code is that its accuracy and cost can be tuned in a wide range, from quick exploratory calculations to highly accurate simulations matching the quality of other approaches, such as plane-wave methods.