Stilfragen: Grundlegungen zu einer Geschichte der Ornamentik is a book on the history of ornament by the Austrian art historian Alois Riegl. It was published in Berlin in 1893. The English translation renders the title as Problems of style: foundations for a history of ornament, although this has been criticized by some. It has been called "the one great book ever written about the history of ornament."

Riegl wrote the Stilfragen while employed as director of the textile department at what was then the Österreichisches Museum für Kunst und Industrie (today the Museum für angewandte Kunst) in Vienna. His primary intention was to argue that it was possible to write a continuous history of ornament. This position is argued in explicit opposition to that of the "technical-materialist" school, according to which "all art forms were always the direct products of materials and techniques" and that ornamental "motifs originated spontaneously throughout the world at a number of different locations." Riegl associates this view with the followers of Gottfried Semper, who had advanced a related argument in his Der Stil in den technischen Künsten; oder praktischer Ästhetik (Style in the technical arts; or practical aesthetics, 1878–79). However, Riegl consistently disassociates Semper's followers from Semper himself, writing that

The theory of the technical, materialist origin of the earliest ornaments is usually attributed to Gottfried Semper. This association is, however, no more justified than the one made between contemporary Darwinism and Darwin.

As the technical-materialist position had attained the status of dogma, Riegl stated that "the most pressing problem that confronts historians of the decorative arts today is to reintegrate the historical thread that has been severed into a thousand pieces." Accordingly, he argued for a continuous development of ornament from ancient Egyptian through Greek and Roman and up to early Islamic and, eventually, Ottoman art.

The Stilfragen is divided into an introduction, which sets out the purpose of the work, and four chapters, each on a theme in the history of artistic style.

The first chapter, "The Geometric Style," argues that geometric ornament originated, not from such technical processes as wickerwork and weaving, but rather from an "immanent artistic drive, alert and restless for action, that human beings possessed long before they invented woven protective coverings for their bodies." He supported this position through an analysis of geometric ornament in Stone Age European art, in particular objects that had recently been discovered in the Dordogne. This ornament, he argued, developed from attempts to represent natural forms in two dimensions, which gave rise to the idea of an outline. After this "invention of line," the cave-dwellers proceeded to arrange lines "according to the principles of rhythm and symmetry."

The second chapter, "The Heraldic Style," addresses compositions of "paired animals arranged symmetrically to either side of an intervening central element." This type of decoration had been associated by previous scholars, most notably Ernst Curtius, with the technical demands of silk-weaving. Riegl argued instead that heraldic ornament arose before the invention of mechanical weaving-looms, and stemmed from a desire for symmetry.

The third chapter, "The Introduction of Vegetal Ornament and the Development of the Ornamental Tendril," traces an unbroken evolution of vegetal ornament from ancient Egyptian through to late Roman art. Here Riegl argues that motifs such as the lotus flower, although they may originally have been endowed by the Egyptians with symbolic significance, were adopted by other cultures that "no longer understood their hieratic meaning," and thereby became purely decorative. In the most famous section of this chapter, Riegl argued that acanthus ornament was not derived from the acanthus plant, as had been believed since the time of Vitruvius, but was rather a sculptural adaptation of the palmette motif. It was therefore "a product of pure artistic invention," and not of "a simple compulsion to make direct copies of living organisms."