Advances in the Theory of Chemical and Physical Systems is a collection of 26 selected papers from the scientific presentations made at the 9th European Workshop on Quantum Systems in Chemistry and Physics (QSCP-IX) held at Les Houches, France, in September 2004. This volume encompasses a spectrum of developing topics in which scientists place special emphasis on theoretical methods in the study of chemical and physical properties of various systems: Quantum Chemical Methods (including CC and DFT for excited states) Relativistic and Heavy-Element Systems (including radiative and nuclear effects)Complexes and Clusters (including metal complexes and clusters) Complex Systems (including quasicrystals, nanotubes and proteins).