Quantum chemistry and solid-state theory are two important related fields of research that have grown up with almost no cross communication. This book bridges the gap between the two. In the first half, new concepts for treating weak and strong correlations are developed, and standard quantum-chemical methods, as well as density functional, Green's function, functional integral, and Monte Carlo methods are discussed. The second half discusses applications of the theory to molecules, semiconductors, homogeneous metallic systems, transition metals, and strongly correlated systems such as heavy-fermion systems and the new high-Tc superconducting materials.