Plant and soil nematodes may be both abundant (>10 million m-2) and diverse (> 90 species) in a particular agricultural production system. Although for centuries considered as the plant pathogens they are now, through their effects in increasing the availability of plant nutrients, also known to make positive contributions to ecosystem processes. The plant-pathogenic aspects of nematodes in temperate, subtropical, and tropical agricultural systems are reviewed in Evans et al. (1993) and Luc et al. (1990). Some nematodes have alternate life-cycles with a bacterial-or fungal-feeding life-cycle contrasting with one parasitic in invertebrates; such species may play important roles in regulating populations of pest invertebrates in agricultural production systems. Nematode parasites of grazing animals also occur in agricultural systems but, although some (e.g., Trichostrongylidae) have bacterial-feeding juveniles in soil and dung, they are not included in this chapter, which focuses on nematodes of cropping, horticultural, and pastoral systems.