This book presents a comprehensive overview of density functional theory (DFT), from its basics to its practical application and implementation. It also discusses the breakthroughs in the field and the complete integration of physical and chemical aspects. It examines both orbital and time-dependent functions along with their variations according to semiquantitative analysis. The book also discusses analytical and computational techniques and principles, considering the classical and quantum approaches. Also covered are important topics such as HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital), MEP (minimum energy paths), KS-DFT (Kohn-Sham density functional theory), UHFD (Unrestricted Hartree-Fock-Dirac), and Gaussian methods.