Weeds are a fascinating study for specialists, not only because of their economic importance, but also since in this case biology must be combined with history and agriculture (and its economic aspects). Thus, weed scientists may be concerned with pure basic research, concentrating on general aspects, or with applied science, i.e. having a practical orientation. One of the aims of this book is to create a synthesis between these two branches of study and to review the literature of both fields. The agrestals, the weeds of arable land ~ the most important group from an economic point of view ~ was chasen as the main topic. Other weed groups could only be mentioned briefly (e.g. grassland weeds), or superficially (e.g. aquatic weeds), or had to be omitted completely (e.g. ruderals, because they are so heterogeneous), to keep this volume to an acceptable size and price. Nevertheless, nearly all subsections of botanical science have been treated.