The term “ornament” has been – and still is – a true topos in the definition and description of so-called “Islamic” art. It is essential to shed a critical light on this position: pre-scientific studies on ornament by theoreticians from the 19th century, such as Owen Jones, are at the origin of an essentialist and taxonomic approach to ornament, which, via Alois Riegl’s method of the history of styles, also had a profound impact on 20th-century art. Specialists of Islamic art such as Ernst Kühnel and Titus Burckhardt carried on this approach that regarded Islamic art mainly from a formal perspective or as a universal constant. Only one new approach has been forged over the past decades. Oleg Grabar has been foremost in considering ornament as a vehicle for expression and messages, basing his reflection on ideas borrowed from the psychology of perception and the history of reception as introduced into art history mainly by Arnheim and Gombrich. More recent research, by scholars such as Gülru Neçipoğlu and Valérie Gonzalez, has highlighted and broadened this position by emphasizing an intellectualized perspective on ornament and geometry.The present article shares the principal positions of these new approaches to ornament in Islam. But in doing so, it focuses on the communicative and expressive qualities of works of art covering a broad formal spectrum. The argumentation is developed anhistorically through several important examples drawn mainly from the first centuries of Islamic rule, beginning with the Dome of the Rock and its marble and mosaic decoration, and with the woodwork from the al-Aqsa mosque, whose rich and powerful vegetal designs directly express the triumphal climate of this period of empire-building. Other examples, such as the ornate façade of the Mshatta palace, the stucco-work at Samarra, and Fatimid ivories, show the different artistic expressions of a supposedly decorative art, expressions that defy universal and taxonomic interpretive models and that are far more that simple decorative ingredients or a surface phenomenon. It is therefore the term of ornament itself that must be examined. It would be more pertinent to raise the question of “ornamentology”, which, parallel to iconologie, might be better suited to the characteristics of this variety of artistic expression.