The rust fungi or Uredinales are an important group of plant parasites, causing great losses to agricultural and forest crops each year. The life cycle of a typical rust is complex, as live types of spores are produced on two different and unrelated hosts, certain of the spore types being always borne on one host and the remainder on the other. Occasionally, however, the life cycle is incomplete, and the fungus needs only one host. The production of each different spore form constitutes a separate stage in the life cycle of the rust, and each spore form, also, performs a specialised role. The mycelium of the fungus may be perennial, living in the host tissues and producing spores for a number of years, or it may live for less than a year, its existence coming to an end with the death of its host, or the death of that part of its host on which it has been subsisting. This booklet provides information on the majority of rusts affecting British forest trees.