This book starts from the most elementary ideas of molecular orbital theory, and it leads the reader progressively towards an understanding of the electronic structure, of the molecular geometry and, in some cases, the reactivity of transition metal complexes. The use of simple notions, such as symmetry, overlap, and elec- tronegativity, allows a qualitative method of analysis of the electronic structure of complexes, and of the properties which follow from it such as geometry or reactivity, to be developed. Qualitative in the sense that, for example, it enables us to understand why the structure of a particular complex is tetrahedral rather than planar, without being able to provide a reliable numerical value of the energy difference between these two structures.