similarities in chemical reactivity depend on molecular properties, and are ultimately dependent on the similarities of electronic structures. fundamentally, quantum chemical similarities are manifested in similarities of molecular behaviour. this book covers both the quantum chemical origins and the methods of phenomenological descriptions of molecular similarity. the emphasis on reactivity is a unique feature. the exposition of computational methods and the prediction of reactivities, as well as the description of actual computer programs constitute important aspects of the book. specific applications in drug design and techniques for the interpretation of the roles of functional groups in reactivity are of interest in molecular engineering. the selection of topics provides a detailed and balanced introduction to the field of similarity-based assessment of chemical reactivity. for researchers and graduate students in both fundamental chemistry and applied fields, such as biochemistry, pharmacology and drug design.