In a classical biological control programme, the selection of the most efficient and safest natural enemy species is crucial. This chapter discusses the criteria used to choose parasitoids and predators for introduction to control invasive arthropod pests. These selection criteria have evolved over time and currently prioritize the natural enemy’s ability to control the pest in its area of origin or in other introduction areas, as well as its specificity for its host or prey to avoid non-target effects. When selecting natural enemies for introduction into a new area, simply opting for natural enemies based on availability in laboratory cultures elsewhere may lead to the introduction of poorly adapted species. Other criteria should be considered, such as climate suitability, the natural enemy complex in the region of introduction, and the presence of cryptic species and biotypes with different bioecological characteristics. Many biological traits of natural enemies have been cited to contribute to successful establishment and control. However, analyses showed that few of them clearly influence the success of biological control. Therefore, efforts should also focus on the traits of the target species, abiotic factors, and biological control approaches and strategies.