The phenomenon of the halogen bond has been known for over a century. In the paper ‘The halogen bond in solution’ (Chem. Soc. Rev., 2012, 41, 3547), M. Erdelyi reviews the current state of knowledge of the nature of the halogen bond. Describe what is meant by halogen bonding. Work on the halogen bond received the 1969 Nobel Prize in Chemistry. Who were the recipients of this prize? Describe the relevance of halogen bonding in biology. List the techniques that have been used to probe the nature of halogen bonded interactions and outline the molecular orbital description of such a bond. Explain what is measured by the diiodide basicity scale and give the assumptions on which the scale is based. Give the reference in which this scale was first mentioned.