Volume II/26 supplements the previous compilations II/l, II/9 and II/17 of the magnetic properties of free radicals which were published in 1965, 1977-1980 and 1986-90. In the form of books and CD ROM it covers the literature from about 1985 to 2001. 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, nitrenes, etc. As before the species have been grouped according to chemical aspects. The contents of the individual subvolumes are indicated on the inside of the front covers. For each group of substances the literature has been compiled and extracted by experts in the fields. A small overlap between the chapters is intentional and allows a maximum of coherence and comprehensiveness of the display. For the reader's convenience an index of substances follows in the last subvolume. Data retrieval is also facilitated by helpful links in the CD ROM version. We wish to thank all the authors for their careful and experienced work and the most agreeable cooperation, the Landolt- Boernstein office, especially Mrs. A.