The role the Handbook of Computational Chemistry is threefold. It is primarily intended to be used as a guide that navigates the user through the plethora of computational methods currently in use; it explains their limitations and advantages; and it provides various examples of their important and varied applications. This reference work is in two parts. The first part consists of volume 1 which briefly describes different methods used in computational chemistry. Basic assumptions common to the majority of computational methods based on molecular, quantum, or statistical mechanics are outlined. Particular attention is paid to the limits of their applicability. The second part consists of a series of volumes exemplifying important applications of computational chemistry. Volume two describes applications of computational methods to model systems. It discusses in detail, molecular structures, the modeling of various properties of molecules and chemical reactions. Both ground and excited states properties are covered in the gas phase as well as in solution. Nanomaterials are described in volume three. Topics include clusters, periodic, and nano systems. Special emphasis is placed on the environmental effects of nanostructures. Volume four is devoted to an important class: Biomolecules. It provides useful models of biological systems considered by computational chemists. RNA, DNA and proteins are discussed in detail. Examples are given for calculations of their properties and interactions. The role of solvents in biologically important reactions is revealed, as well as the structure function relationship of various classes of Biomolecules.