The chemical ecology of Oribatida is tightly integrated with a distinct exocrine system in the opisthosoma, known as ‘oil glands‘ (syn. opisthonotal glands). Representing homologous structures, oil glands characterize the four morederived cohorts of Oribatida (Parhyposomata, Mixonomata, Desmonomata, and Brachypylida), but also theAstigmata, as the monophyletic unit of ‘glandulate Oribatida’. Generally, oil glands constitute large intima-lined sacsthat are located in the dorso-lateral regions of the idiosoma and that open to the body outside via a single (frequentlyflapped) pore on either side of the notogaster. Secretions of more than 20 oribatids have so far been analyzed. Theyconsist of hydrocarbons, terpenes, aromatics, and alkaloids. Many components occur in specific combinations; secretionprofiles characterize groups (on any taxonomic level) and have emerged as tools for phylogenetic analyses:Parhyposomata, e.g., produce phenolic- and naphthol-rich secretions, whereas a distinct set of terpenes and aromatics(the so-called ‘astigmatic compounds’) is considered synapomorphic for middle-derived Mixonomata and allgroups above (‘astigmatic compounds-bearing Oribatida’). In some subgroups of the ‘astigmatic compounds-bearingOribatida’, these components are not easily traced as they tend to be reduced and replaced by others. Functionally,oil glands produce various allomones against predators and fungi, and alarm pheromones for intraspecific communication. Pheromonal properties of oil gland compounds probably evolved early in ancient oil gland-bearing oribatids from purely defensive functions, culminating in a radiation of semiochemical roles (alarm, aggregation, sex) in oil glands of the Astigmata.