Volume II/26 supplements the previous compilations II/l, II/9 and II/17 of the magnetic properties of free radicals. Due to the still rapid growth of the field and the necessary inclusion of new subjects the volume is divided into subvolumes which will appear in fast succession. Together with the earlier publications volume II/26 offers an up-to-date and comprehensive survey and collection of structures and data on the important chemical intermediates, namely radicals, polyradicals and related species such as carbenes. As before the species have been grouped according to chemical aspects. For each group of substances the literature has been compiled and extracted by experts in the fields. For the readers convenience an index of substances follows in the last subvolume.