Weeds must be considered in any conceptual framework that seeks to explain, explore and analyse biodiversity if the focus is on human activities and utilisation of natural systems. This chapter concentrates on weeds in annual and perennial crops associated with terrestrial systems used for agricultural purposes: weeds of non-terrestrial or nonagricultural sites are not considered. Genetic diversity within weed species; weed species diversity in the context of conservation; and the particular role of weeds and their seeds for food webs are discussed. The size of seed-based food webs in agroecosystems is largely determined by the amount of energy produced by weed seeds. The chapter describes how aspects of weed biodiversity, besides the conservation of rare species, can be used to induce, guide or evaluate sustainable development in arable production. The local weed species diversity is shaped by both site conditions and arable management adapted to those conditions.