Bond valence theory is a classical theory of chemical structure based on the concepts of atoms and bonds. It complements quantum theory, each theory providing different information about chemical structure. The first section develops the physical theory behind the model and the various derived theorems and rules that have general application. The second section addresses more specialized aspects such as the properties of lone pairs and hydrogen bonds. The third section contains a number of worked examples to illustrate the application of the theory while the fourth section is brief introduction to aqueous solutions.