The. first edition of this work appeared almost thirty years ago, when, as we can see in retrospect, the study of the actinide elements was in its first bloom. Although the broad features of the chemistry of the actinide elements were by then quite weil delineated, the treatment of the subject in the first edition was of necessity largely descriptive in nature. A detailed understanding of the chemical consequences of the characteristic presence of 5f electrons in most of the members of the actinide se ries was still for the future, and many of the systematic features of the actinide elements were only dimly apprehended. In the past thirty years all this has changed. The application of new spectroscopic techniques, which came into general use during this period, and new theoretical insights, which came from a better understanding of chemical bonding, inorganic chemistry, and solid state phenomena, were among the important factors that led to a great expansion and maturation in actinide element research and a large number of new and important findings. The first edition consisted of aserial description of the individual actinide elements, with a single chapter devoted to the six heaviest elements (lawrencium, the heaviest actinide, was yet to be discovered). Less than 15 % of the text was devoted to a consideration of the systematics of the actinide elements.