AbstractEstablishment of exotic natural enemies in the area of release is not a desirable attribute in inundative releases as it increases the risks of non-target effects on native species. To evaluate the risks of non-target effects, this chapter focuses on factors which may limit the establishment of introduced natural enemies, either for a season or permanently. From a risk assessment perspective, the risk associated with the release of a species with seasonal persistence capacity is limited in time. The establishment of natural enemies in a novel habitat depends on several factors, some abiotic and some biotic. Among the abiotic factors, climate is a major factor. Temperature and humidity, especially when soil moisture is considered in species that spend part of their development in the soil, are the components of weather that have the major impact on the survival and establishment of exotic species. Biotic factors, and especially the occurrence of alternate host/prey, also play an important role in the probability that an organism will become established. We describe in this chapter the methods that should be used to assess the probability that exotic natural enemies can become established, based on these factors. We recommend first evaluating to what extent temperature may limit establishment. Only where the risk of establishment based on thermal requirements is determined to be higher than 'insignificant', should the availability and suitability of host or prey for overwintering in the non-target habitat or the impact of humidity be investigated.