Density functional theory (DFT) has blossomed in the past few decades into a powerful tool that is used by experimentalists and theoreticians alike. This book highlights the extensive contributions that the DFT-based OLCAO method has made to progress in this field, and it demonstrates its competitiveness for performing ab initio calculations on large and complex models of practical systems. A brief historical account and introduction to the elements of the theory set the stage for discussions on semiconductors, insulators, crystalline metals and alloys, complex crystals, non-crystalline solids and liquids, microstructure containing systems and those containing impurities, defects, and surfaces, biomolecular systems, and the technique of ab initio core level spectroscopy calculation.