This text presents a unified and up-to-date discussion of the role of atomic and molecular orbitals in chemistry, from the quantum mechanical foundations to the recent developments and applications. The discussion is mainly qualitative, largely based on symmetry arguments. It is felt that a sound mastering of the concepts and qualitative interpretations is needed, especially when students are becoming more and more familiar with numerical calculations based on atomic and molecular orbitals. The text is mathematically less demanding than most traditional quantum chemistry books but still retains clarity and rigour. The physical insight is maximized and abundant illustrations are used. The relationships between the more formal quantum mechanical formalisms and the traditional chemical descriptions of chemical bonding are critically established. This book is of primary interest to undergraduate chemistry students and others taking courses of which chemistry is a significant part.