The present chapter offers an overview of polyanions of the Group 17 as examples of catenation. The main knowledge obtained from the rich literature on polyiodides is summarized, and comparisons are made with other polyhalide systems in order to gain insights into similarities and differences. A strong emerging field is represented by polybromides in the liquid and solid state, as well as novel or rejuvenated areas of application. A detailed analysis of chemical bonding is given, and the special properties of halogen-halogen bonding are highlighted.