Hello, and welcome to this introductory course on density functional theory, also called DFT. DFT is one of the many ways to tackle the many-body quantum problem. It is a rigorous theoretical framework for which efficient approximations exist and make it suitable for large-scale calculations. Indeed, it is today and by far the most used approach for electronic structure calculations in material science. The applicability of density functional spans from biology to molecular chemistry, from nuclear physics to Stellar Materials and astrophysics. The reasons for this success are multiple. You should understand them during the course. This is a three-week course, so it is not so short introduction to density functional theory. The objectives are to understand the main ideas behind the concept of functional of the density, especially in the context of the many-body problem to get detailed insight of the mathematical and also historical foundation of DFT, to get an idea about strategies that have been used and are still used to design approximations, to learn what is specific to the many different approximations, and to get an idea how they perform for a given material and observable, finally, to get an idea of the practical procedure to solve the problem using DFT. At the end of this introductory course, if you wish, you should be able to go beyond on one side, to tackle more advanced theoretical concepts. On the other side, to become more productive and really use DFT for your own research with the approach and computer code of your choice. We do not require much prior knowledge to follow this course. You should just know quantum mechanics, of course. The rest will be introduced and explained when needed. To help and encourage you all along, a series of exercises are available, do not miss them for they are the best way to consolidate concepts and understanding. On behalf of all tutors, I wish you all a good density functional time.