I would like to present to a wide circle of the readers working in quantum chem- istry and solid-state physics, as, -ell as in other fields of many-body physics and its interfaces, this book deyoted to density functional theory written by my colleagues Eugene S. Kryachko and Eduardo Y. Ludena. Their ways to this theory are rather different although basically both of them are quantum chemical. Eugene S. Kryachko came to energy density functional theory from the theory of reduced density matrices, and Eduardo \'. Ludena dewloped earlier the concept of loges in quantum chemistry. Neyertheless, their earlier interests giw the possibility to consolidate and formulate energy density functional theory in a unified and consistent way, in my opinion. Raymond Daudel Paris ACKNOWLEDGMENTS The authors are indebted to Carl Almbladh, Victor Va. Antonchenko, John Avery, Richard F. W. Bader, Ulf \'on Barth, Jean-Louis Calais, A. John Coleman, Jens P. Dahl, Robert Donnelly, Harold Englisch, Robert 1\1. Erdahl, Oswaldo Goscinski, John E. Harriman, Gintas Kamuntavichius, Illja G. Kaplan, Jaime Keller, \'alentin Khart- siev, Toshikatsu Koga, Per-Olov Lo\ydin, T. Tung Nguyen-Dang, Ivan Zh. Petkov, Jerome K. Percus, l\lary Beth Ruskai, John R. Sabin, Zdenek Slanina, \'ladimir Shi- rokov, l\lario V. Stoitsov, Yoram Tal, and \Vaitao Yang, who in one way or another, either through their kind support, help, discussions or valuable comments created the human and intellectual background which made this book possible.