Mme. Curie discovered polonium as a part of her doctoral thesis research. Polonium was the first element to be discovered solely because of its inherent radioactivity. Because there was no knowledge of polonium's chemical properties, the separation steps were followed using only the increase in the specific radioactivity. The first Gmelin Polonium volume, edited in 1941, shows how little was known about polonium for a long time. More detailed investigations on this element were performed only after World War II when the handling of large quantities of highly radioactive nuclides had been developed. Most information on this heaviest Main Group VI element comes from investigations on the tracer scale, especially data that describe its behavior in solution. On the other hand, many solid compounds, such as halides, oxides, and metal polonides as well as the metal itself, are produced in weighable amounts so that structural data are also available. However, these data are far from complete so there is a wide-open field for future research, especially by using the newly developed ultramicrotechniques for preparation and identification of radioactive compounds in the nano- to microgram range. The present volume of the Gmelin Handbook, Polonium Suppl. Vol. 1, gives a very clear, critical, and updated picture of the different aspects of the behavior of polonium - from chemical and physical properties to production, from the nuclear properties of the 29 isotopes to its biological and environmental behavior, and finally to the use of polonium in a variety of applications. Because of the interest in this element has decreased markedly in recent years (apart from its environmental behavior), this volume will be a special and unique source for polonium information for a long time. It also clearly shows that large gaps in our knowledge of this rare natural radioelement still exist. Consequently the volume can serve as a guide to future research initiatives.