Though Density Functional Theory (DFT) is no longer novel, it still is a rapidly developing area. A sign of its combined vigor and maturity in recent years has been the widespread application of DFT to the study of the structural and electronic properties of all kinds of materials of interest in physics, chemistry, and materials science. More recently, the study of biologically important problems via DFT has grown extremely rapidly as well. Widespread recognition of DFT as a remarkably appropriate method for the theoretical study of such a variety of problems and materials is due to the combination of two characteristics. First, the one-electron character of the Kohn-Sham (KS) equations, and the fact that all the complications of inter-electronic exchange and correlation effects are embodied in an effective potential appearing in those KS equations, have the consequence that large systems can be handled much more easily than by other many-electron (so-called “ab initio” or quantum chemical) methods. Second, the degree of accuracy achieved with present-day approximations to the exchange-correlation energy functionals and potentials is sufficient for many purposes. For these reasons, diverse groups have developed powerful DFT codes that enable realistic computer simulations of an enormous variety of problems of practical interest. Nonetheless, there are outstanding issues remaining for even the ground state theory, e.g., better functionals and more rational routes to them. For excited states, dynamics, and for high magnetic fields, the range of unsolved problems is considerably broader. But the success of the ground state theory gives reason for optimism, hence the whole subject continues to have a notable and characteristic vitality. It also has the unusual and generally stimulating aspect of bringing together theoretical and computational chemists and physicists. This special issue of International Journal of Quantum Chemistry presents the Proceedings of the 9th International Conference on the Applications of the Density Functional Theory to Chemistry and Physics. The Conference was held September 10–14, 2001, in El Escorial, a village in the mountains near Madrid of great importance in Spanish history. Having the 9th Conference in El Escorial is but one manifestation of the present strong interest on DFT in Spain. That interest was initiated around 1968 by E. Santos, then at the University of Valladolid, who worked on the interaction between graphitic layers and also on the surface of atomic nuclei. Since that time many other Spanish scientists have joined the DFT community. The 9th Conference was attended by over two hundred participants, and included plenary, invited and contributed talks, organized in topical sessions. Topics included: Fundamental aspects of DFT, Applications to Chemistry, Applications to Physics, Van der Waals Interactions, and Complex Systems. Three lively poster sessions were held at the open patio of Colegio Universitario María Cristina. The concluding remarks were presented by Eduardo Ludeña, who did a superb job in summarising the new developments presented at the Conference and in looking towards the future. The papers included in these proceedings were subject to the usual refereeing procedure. With only a few omissions, they represent well the breadth and variety of Conference presentations. This biennial series of Conferences included, most recently, those held in Paris, Vienna, and Rome (1995, 1997 and 1999 respectively). The next one will be in Brussels, Sept. 5–12, 2003. P. Geerlings (Brussels) is heading the local arrangements. We are happy to express gratitude to the Organizing and Local Committees for the help and the facilities provided, and also to the sponsors of the Conference: Universidad Autónoma de Madrid, Universidad Politécnica de Madrid, Escuela Superior de Ingenieros de Telecommunicación, Universidad de Cantabria, Spanish Ministry of Science and Tecnology, European Physical Society, Cost Action D9 Program of the European Community, and Psi-K Program of the European Science Foundation. We also thank Ms. Arlene Rodriguez for her work as Publications Assistant and N. Y. Öhrn and J. R. Sabin for the opportunity to serve as Special Editors.