This chapter summarizes the occurrences, atomic structures, and molecular properties of the elements of group 17, chlorine, bromine, iodine, and astatine. The charge‐transfer complexes of halogens with different inorganic or organic compounds are outlined. The interhalogen compounds, especially those of polyiodides, are discussed with respect to their preparation and their structure varieties. The halogen oxides such as halogen oxide cations, halogen oxide fluorides, oxoacids and oxoacid salts, and halogen derivatives of oxoacids are discussed. Finally, organic polyvalent halogen derivatives, mainly including those of iodine (I), iodine (III), and iodine (V), are mentioned.