Recognition and discrimination ability of organisms is assumed in numerous biological and ecological contexts. Recognition systems research has originally focused on intraspecific interactions and social recognition but is similarly applicable to interspecific interactions and recognition of environmental features. Common features of recognition systems are at least two participants, called cue bearer (sender) and evaluator (recipient), and three components, expression, perception, and action (Sherman et al. 1997, Liebert & Starks 2004, Mateo 2004, Starks 2004). The recognition systems concept provides a unified framework for studying and understanding recognition processes across biological and ecological contexts. In general, recognition may be indirect, mediated by third organisms or environmental features of the cue-bearer, and direct. I here focus on the latter type of recognition and emphasize how direct recognition processes govern intraand inter-specific interactions within and between plant-inhabiting arthropod species and the plant they inhabit. All species dealt with have relevance in agriculture as either herbivorous pests or natural enemies. Each studied system represents a subset of a multi-trophic community that may occur in natural settings and may be artificially created in greenhouse crops in the course of biological control (Figure 1). The interacting organisms in this community are the predatory mites Phytoseiulus persimilis and Neoseiulus californicus, their prey, the herbivorous two-spotted spider mite Tetranychus urticae and western flower thrips, Frankliniella occidentalis, the host plant of the herbivores, common bean, Phaseolus vulgaris, and a below-ground symbiont of bean, the mycorrhizal fungus, Glomus mosseae. Highlighted topics include mycorrhiza-induced host plant quality recognition by T. urticae (Hoffmann et al. 2008), recognition of and preference towards T. urticae life stages by P. persimilis (Blackwood et al. 2001, 2007), predator recognition and within plant dispersion of T. urticae (Walzer et al. 2008), predator recognition by F. occidentalis and trait-mediated predator effects (Walzer & Schausberger 2008), early learning and recognition of F. occidentalis by N. californicus (Schausberger et al., 2008), intraguild predator recognition with emphasis on P. persimilis and N. californicus (Schausberger & Croft 2000ab, Schausberger & Walzer 2001, Walzer & Schausberger 1999, Walzer et al. 2006), and kin recognition and cannibalism by P. persimilis (Schausberger & Croft 2001; Schausberger 2005, 2007; Schausberger & Hoffmann 2008). The organizational level worked with in the aforementioned studies primarily was the individual level but also included populations and communities; the experimental scale ranged from artificial cages, to detached leaf cultures and whole plants. For each recognition system and ecological context I emphasize the adaptive significance of recognition and indicate, if applicable, the potential population consequences of a given recognition process. In conclusion, I argue that recognition systems are ubiquitous in the studied multi-trophic community and point at the relevance of recognition systems research to biological control.